Abstract

In this paper, the elephant in the room is the issue of projected longer-term population growths and declines in a finite world, while the Greek goddess Panacea is the opportunity to non-disruptively attain populations that live sustainably and solve some of the world’s most pressing problems. Particularly addressed in this paper is the potential of water systems models and modeling to facilitate a transition to planning for long-term sustainable lives of sustained quality. Anent “longer term”, for simplicity we consider both a typical period of family memory, for instance from 1880 to 2100, or 220 years, about seven or eight generations, as well as the anthropocene millenia.

After setting down background definitions and introducing the underlying issues, we review key population and well-being trends, attitudes, and impacts, citing acknowledged experts. How, where, and when population and economic decline will occur is not covered; the paper rather suggests implications for water resources engineering and for water management modeling, even if the transition will be patchy in space and time. Confronting imminent degrowth, significant revisions of current water modeling practice are suggested: planned, phased, orderly removal of projected and existing urban development and drainage infrastructure and, for instance and where applicable, systematic restoration of keystone ecology and natural hydrology. Whether the imminent degrowth era will persist is uncertain, evidently. Also alluded to is degrowth’s countervailing assurance of improved well-being, providing more time for individuals to further their personal interests.

The original PowerPoint presentation is at Robillynians.org and also at the CHI website (James 2023). Questions raised and the answers given at the presentation are included in the appendix to this paper.

Preliminary definitions

In this section, concepts not commonly considered in the subdiscipline of water systems management modeling, but key in this paper, are listed and ellucidated.

Degrowth, décroissance: Degrowth broadly means shrinking rather than growing economies, using less of the world's energy and resources and putting wellbeing ahead of profit. The idea is that by pursuing degrowth policies, economies can help themselves, their citizens, and the planet by becoming more sustainable (Wikipedia 2022). Degrowth allows people to spend more time on their personal interests and pursuits, much like taking early and long retirements.

Sustainability: Sustainability incorporates the processes and actions through which humankind avoids the depletion of natural resources and maintains the quality of life of modern societies. The Brundtland Report (World Commission on Environment and Development 1987) defined sustainable development as "development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs" (Wikipedia 2023c) The practicality of this definition has been widely criticised.

Quality of life: Components of the quality of life include Health, Education, Environmental Quality, Personal Security, Civic Engagement, and Work-Life Balance. In this paper, for simplicity, they are subsumed by Life Expectancy (Wikipedia 2023d). Statistics for these measures are readily available on the web.

Life expectancy: Life expectancy is the number of years a person can expect to live and is an estimate of the average age that members of a particular population group will be when they die. In Canada, it has increased from 44 in 1880 to 83 today (2022) and is expected to be 92 in 2100. For men and women, healthy lives were 5 years and 6 years shorter, respectively. Mean length of retirement for men increased from 14 years in 1976 to 20 years in 2019, while for women, mean retirement is 5 to 6 years longer (Bloomberg News 2023).

Well-being: Well-being is determined by social, economic, and environmental conditions and covers quality of life. Focusing on well-being supports the equitable distribution of resources, overall thriving, and sustainability. A society’s well-being can be determined by the extent to which members are resilient, build capacity for action, and are prepared to transcend challenges. (World Health Organisation 2023). It can be quantified and is a more humane alternative to Gross Domestic Product (GDP). Well-being is exemplified by the increased length of retirement over the past few generations.

Resilience: Resilience is the process and outcome of successfully adapting to difficult or challenging life experiences, especially through mental, emotional, and behavioral flexibility, and adjustment to external and internal demands. Being resilient includes competence, confidence, connection, character, contribution, coping, and control (Wikipedia 2023e).

1 Introduction

In a recent presentation (James 2023) the author introduced ideas that challenge precepts such as “growth is good” and “our lifestyle is sustainable”. Collected over 66 years of academic engineering practice at ten universities, ten propositions were presented (an eleventh is asterisked in Section 6); the ten listed here are not articles of faith but positions to be thoughtfully examined, à la the method of Socrates; they are presented here because they fostered lively debate in the author’s lectures:

1. There is no water scarcity anywhere, anytime; there is no less, nor more water on earth than has always existed in our current geological era—there is rather a surfeit of people (given local flow variability). Stating the problem this way opens a broader set of long-term sustainable solutions, rather than the usual set of unsustainable short-term fixes.
2. Most water-related issues are caused by people, and are reduced by reducing the number of people (this assumes that people make informed decisions about the risks where they are and in what they are doing at any time); at this point in history exceptions to the rule might include currently unpredictable cataclysms, currently unsourced diseases, etc. In other words, lack of knowledge contributes. A corollary to this rule is that a purpose of Science is to explain how humans are responsible for their water problems and predicaments.
3. We are unlikely to resolve acute water problems permanently; one may replace them with chronic problems and the cost is vigilance; thus water systems design is merely a provisional lash-up. Nevertheless, problems are opportunities—Water Resources Engineering is a rewarding calling, where vigilance is axiomatic.
4. All issues boil down to Physics; blind allegiance to codes of practice, or procedure, or belief, or loyalty, or devotion, or conviction, or faith is the antithesis of knowledge, which is to say, ignorance. Unsettling beliefs are worse: stupidity. Here we include the unscientific belief that there are no limits to growth in a finite world.
5. Chaos is the natural and best solution; by “chaos” the writer means increasing entropy, the complex relationship between members of diverse ecosystems; entropy increases naturally, and its detailed impact is often uncertain. Seek to maximize biodiversity.
6. In the end, the movement or transport of resources (including water), and of objects and people, is unsustainable. Short distances are better, however (as is walking and biking, for instance).
7. A practical and overdue step in the resolution of “populution” [obnoxious hybrid word] is empowerment of women by education to the highest levels. This conclusion simply follows Occam’s razor, is inherently kind, and manageable.
8. The key to long-term serenity is caring for others and later generations, which includes caring for their environment. Happiness is the absence of pain of both kinds, physical and mental. Being responsible for animal cruelty, for example, is an easily avoided pain, for all involved – trend towards veganism, if only because the environmental, health, and animal cruelty benefits are huge. This proposition may seem tangential but is not.
9. Elementary degrowth has been practiced by Water Resources Engineers for more than a century, witness dam removals in New England, sewer daylighting in many large cities, restoration of wild areas from saline agricultural land, re-establishment of wetlands, etc. But it has been disjointed and proceeded at a snail’s pace.
10. “Degrowth” (meaning prosperity without growth) delivers improved quality of life, and more time for enjoying one’s own mix of hobbies and activities (other than the race to affluence). Suggestions for improved health and well-being include slowing down (again)—think of early youth and early retirement.

2 Arguments for Growth or Décroissance (“Degrowth”)?

Of course, degrowth concepts are not new, for instance anti-industrialism of the 19th century is discussed by Henry David Thoreau (1817–1862), John Ruskin (1819-1900), and Leo Tolstoy (1828–1910).  The term "degrowth" appeared during the 1970s, proposed and supported by many intellectuals. The Club of Rome funded researchers at the Massachusetts Institute of Technology to produce The Limits to Growth, (Meadows et al. 1972), the first significant study to model the consequences of economic growth. Our Common Future (World Commission on Environment and Development 1987) recognized that human resource development in the form of poverty reduction, gender equity, and wealth redistribution was crucial to formulating strategies for environmental conservation, and that environmental-limits to economic growth in industrialized and industrializing societies existed. In 2019, a summary for policymakers of Global Assessment Report on Biodiversity and Ecosystem Services, the largest, most comprehensive study to date of biodiversity and ecosystem services was published (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services 2019). The main conclusions were:

• Over the last 50 years, the state of nature has deteriorated at an unprecedented and accelerating rate.
• The main drivers of this deterioration have been (i) changes in land and sea use, (ii) exploitation of living beings, (iii) climate change, (iv) pollution, and (v) invasive species. These five drivers are caused by societal behaviors, from consumption to governance.
• Damage to ecosystems undermines 35 of 44 selected UN targets, including the UN General Assembly’s Sustainable Development Goals for poverty, hunger, health, water, cities’ climate, oceans, and land, and can cause problems with food, water, and humanity's air supply.
• To fix the problem, humanity needs transformative change, including sustainable agriculture, reductions in consumption and waste, fishing quotas, and collaborative water management. The report proposes "enabling visions of a good quality of life that do not entail ever-increasing material consumption" as one of the main measures. The report states "Some pathways chosen to achieve the goals related to energy, economic growth, industry and infrastructure, and sustainable consumption and production, as well as targets related to poverty, food security, and cities, could have substantial positive or negative impacts on nature and therefore on the achievement of other Sustainable Development Goals".

The 2019 World Scientists' Warning of a Climate Emergency (Wikipedia 2023f), and its 2021 update (Ripple 2019), assert that economic growth is a primary driver of overexploitation of ecosystems; to preserve the biosphere and mitigate climate change civilization must, in addition to other fundamental changes:

• stabilize population growth,
• adopt largely plant-based diets,
• shift from GDP growth and the pursuit of affluence,
• sustain ecosystems,
• improve human well-being,
• prioritize basic needs, and
• reduce inequality.

Human population raises difficult cultural, psychological, ideological, religious, and ethical issues, and frequent confrontational denials by pro-growth influencers. En passant, we note that the Coronavirus disease 2019  is linked to the ecological crisis (Kolinjivadi 2022; Murányi and Varga 2021). Critiques of degrowth concentrate on:

• the negativity of the term "degrowth",
• the misunderstanding that “growth” is seen as explicitly bad,
• the challenges of a degrowth transition, and
• the entanglement of desirable aspects of modernity with the growth paradigm.

On the other hand, degrowthers use dispassionate, science-based arguments, and their literature has exploded in quantity. An excellent primer is Prosperity without growth (Jackson 2009). Five arbitrarily-chosen responses to pro-growth criticisms are cited below.

First: Population Matters (PM) on 18^th January 2022 reviewed the BBC flagship Radio 4 five-episode ‘Rethink’ series on human population dynamics, impacts, and challenges (Maynard and Scigliano 2022). PM Director Robin Maynard and PM Researcher Monica Scigliano explained that Rethink Population (RP):

• Stressed concerns related to population decline, such as future pressure on pension funds. PM’s report ‘Silver linings, not silver burdens’ (Population Matters 2021a) shows that the concerns are overblown. 
• RP did not reflect that the world is finite and that economic and population growth therefore cannot continue indefinitely.
• Absent from RP was consideration of the environment, even though population is an indirect driver of the climate crisis, mass extinction of wildlife, and depletion of natural resources, according to numerous published scientific reports by expert bodies.
• RP ignored that alt-economists challenge GDP as a true measure of human progress, and focused instead on the more nuanced measure of “well-being”.
• Influencers who dismiss population focus on curbing consumption, rarely considering the drastic decrease needed:
• A 2018 paper (O’Neill et al. 2018) in Nature Sustainability estimated that, given equal usage of resources, our current population can live sustainably if each of us meets only our most basic needs.
• Economist Sir Partha Dasgupta and his daughter, demographer Dr. Aisha Dasgupta (Dasgupta and Dasgupta 2017), put it in terms of GDP: sustainability would only be feasible with a maximum world GDP of USD $70 trillion, which if spread equally among our currently population, would mean less than $9K per person. To put that in context, Guatemala has a per capita GDP of just under $9K, whereas the global average per capita GDP stands at around $20K with Canadian citizens averaging about CAD $52K [WJ] (Our World in Data 2023).
• Any rational concern about the impact of hyper-consumption would also welcome population decline in those parts of the world where consumption is highest.
• Adopting eco-actions such as recycling, cutting out intensively reared meat, driving an electric car, etc., are championed as positive shifts to a more sustainable lifestyle; while indeed beneficial, they make a relatively small impact when compared to the individual eco-action of choosing a smaller family in high-income, high consumption countries.
• A 2017 study by Lund University concluded that having one fewer child could be by far the most effective of all these measures over the long term. 
• Global population would not stand at 8 billion if girls and women everywhere had access to secondary education, sex education, and quality family planning services, and were free from pressure to have large families.
• Suggesting that the solution to economic problems is to have more children is simplistic, erroneous, and harmful. An issue crying out for greater media scrutiny is the shocking rise of coercive pro-natalism, as shown in the report ‘Welcome to Gilead’: women’s rights and choices are increasingly being restricted around the world in attempts to increase birth rates (Population Matters 2021b).

Dr Sarah Harper of the Oxford Institute of Population Ageing provided a welcome voice of reason: “Falling fertility is good: it’s good for the planet, it’s good for communities, and good for women.”

Second: A Toronto lobby group, the Century Initiative, launched a campaign to triple Canada's population to 100 million by the year 2100. Rodrique Tremblay, Professor Emeritus of Economics at the Université de Montréal and former Quebec Minister of Industry and Trade, countered in June 7, 2022 “Why the Century Initiative should be rejected”. Excerpting from what Tremblay wrote [edits are minor – WJ]:
"Canada's population—37 million, 2021 Census—is projected to reach 53 million by 2100 based on normal natural population growth and average immigration policy. However, to double this to 100 million, the Canadian government would have to adopt an ultra-massive immigration policy. With such a demographic scenario, the population of Metropolitan Toronto would increase from 8.8 to 33.5 million, that of Metropolitan Montreal would swell from 4.4 to 12.2 million, etc. It can be expected that that would cause congestion, pollution, overloading of public services in health, education, and transportation infrastructure, ghettoization, language conflicts, crime, insecurity, etc. When a population grows too quickly, it may well be accompanied by a general decline in living standards. … living standards in the world are in no way [positively -WJ] related to the demographic size of countries. The reality is quite the opposite. This is shown by the United Nations Human Development Index, which is a measure of living standards and quality of life around the world. In 2019, for example, the top three countries for living standards and quality of life were all countries with fewer than 10 million inhabitants: Norway (5.3 million), Ireland (5.0 million) and Switzerland (8.5 million). The first main objective of the coalition is to enable the Canadian government to play a greater role on the international stage. Well, a relatively small country like Switzerland is more important in the world than many highly populated countries. The second main objective is to address the ageing of the population and the shortage of labour by increasing Canada’s economic growth rate. However, studies show that immigration does little to change the age structure of a population, mainly because the majority of immigrants arrive in adulthood, and because of the family reunification program, which brings in immigrants who are already elderly (spouses, parents, grandparents, etc.). As for labour shortages in some sectors, mass immigration is leading to an increased demand for labour to build and equip additional infrastructure, which is likely to increase the overall demand for labour. The economy can then face an endless spiral of labor shortages with labor markets permanently strained, artificially created, and inflated by a population that is growing too fast by immigration." [end of paraphrased piece by Tremblay] (Tremblay 2022).

Third: On the other hand, Elon Musk claimed, without supporting evidence, that ‘civilization is going to crumble’ if people don’t have more children, and that low and rapidly declining birth rates are “one of the biggest risks to civilization.” [CNN News report]. Matt Reynolds in WIRED showed that Musk is totally wrong about population collapse by citing Samir KC, a demographer at the International Institute for Applied Systems Analysis (IIASA) near Vienna: if the world’s total fertility rate held at 1.84 babies per woman—the UN’s estimate of what it will be in 2100—the population would fall from 10.4 billion in 2100 to 1.97 billion in 2500, and 227 million in the year 3000, “not exactly a population collapse, but rather a slow-motion population decline. Fixating on global population collapse today is like someone in the year 1000 worrying about the Y2K bug.” (Reynolds 2022).

Fourth: In Europe, John Cody writes that despite EU threats, the Swiss People’s Party (SVP), is pursuing a referendum calling for the renegotiation of international treaties, or even for their complete abandonment, if the Swiss population hits 10 million (Cody 2022). The proposed referendum comes at a time when Europe increasingly faces environmental catastrophe, a housing crisis, and huge strains on public resources due to soaring immigration levels. Over the past 20 years, Switzerland’s population has increased 21%. Many European nations are among the most densely populated nations in the world and life in them is expected to become more crowded in the near future unless dramatic action is taken.

Fifth: China. Wang Feng is a professor of sociology at the University of California, Irvine, and the co-author, with James Lee, of “One Quarter of Humanity: Malthusian Mythology and Chinese Realities, 1700-2000.” Feng is a sociologist who specializes in Chinese demographics. On January 30, 2023 he wrote about the inter-related causes for population decline in an essay “The Alternative, Optimistic Story of Population Decline” (Feng 2023): [excerpts start]
"China, the most populous country on the planet for centuries, this month reported its first population decline in six decades, a trend that is almost certainly irreversible. According to U.N. projections, by 2100 China may have only about half of its 1.41 billion people. The news has been met with gloom and doom, often framed as the start of China’s inexorable decline and, more broadly, the harbinger of a demographic and economic “time bomb” that will strain the world’s capacity to support aging populations. But China is only the latest and largest country to join Japan, South Korea, Russia, Italy, Germany (if not for immigration) and about two dozen others.  In seven decades in the second half of the 20th century, world population more than tripled from 2.5 billion in 1950 to 8 billion in 2022, and the world panicked about unstoppable population growth. Turns out, that was a transitory phase when mortality rates fell faster than fertility rates because of improved nutrition and public health, and relative peace. Population declines in some countries today are largely a happy result of greater longevity and freedom. Global fertility rates have dropped from over 5 births per woman in the early 1960s to 2.3 in 2020, because of greater investment in child and maternal health everywhere – a mother who successfully brings her child to term, and an infant who survives to childhood, lowers birthrates because parents do not need to try again. Greater availability of free or affordable contraception has also reduced unwanted births. China, South Korea, and Japan are now in population decline due in part to rapid increases in income, employment, and education. The number of South Korean women who went on to postsecondary education rose from 6% in 1980 to more than 90% by 2020. Lower birthrates stem in part from greater personal and reproductive freedom, such as the choice to stay unmarried, higher pay, and more professional opportunities for women in these nations. More women in the work force is a recipe for even greater productivity and prosperity and could help ease labour concerns among falling populations. More women than ever are rising to leadership positions in business, media, and politics. Compared to a half-century ago, people in many countries are richer, healthier, and better educated and women are more empowered. China’s population, for example, is shrinking and aging, but its people are more educated and have a longer life expectancy than at any time in the country’s history. Expanded educational opportunities guarantee a spot in a university for almost every person born today in China, including more women than men. Average world life expectancy has increased from 51 years in 1960 to 73 in 2019, and even more so in China, from 51 in 1962 to 78 in 2019. Increases of that magnitude reshape lives and open up opportunities unimaginable when life spans were shorter, such as workers remaining productive later in life, and growing markets for elderly consumers in areas like tourism, nutritional supplements, and medical devices, among others. Fewer people on the planet, of course, may reduce humanity’s ecological footprint and competition for finite resources. There could even be greater peace as governments are forced to choose between spending on military equipment or on elderly pensions. And as rich nations come to rely more on immigrants from poorer countries, those migrants gain greater access to the global prosperity currently concentrated in the developed world. This new demography brings new challenges, including the need to offer quality and affordable child care, make college education more affordable and equitable, provide guaranteed minimum income, and make societies more gender equal. Governments should abandon the mindless pursuit of economic growth in favor of well-being for citizens. There is no reason that world population must keep growing or even remain level. And just as earlier panic led to harmful policies in China and elsewhere, efforts to raise fertility—which may prove futile—risk viewing women once again as birth machines. Global population will inevitably decline. Rather than trying to reverse that, we need to embrace it and adapt." [end of Feng excerpt] (Feng 2023)

3 Time horizon and countries of interest

A typical family memory span is about two centuries, e.g., the writer, born before 1940, his parents about 1910, and grandparents 1880, his offspring ca. 1970, and grandchildren 2000, who themselves may live till 2090 (say 2100 for round numbers). Thus, the collective memory and interest extends from 1880 until (say) 2100, with a generational timestep of 30 years. In 1937, the South African population was 10.3 million (now 59.4 million). Canada (1937: ca.11 million, now 38.3 million) is approximately ten times bigger in area. Life expectancy between the two places is not comparable, 65 years vs. 84 years.

For this paper, countries of subjective interest include Canada, Sweden (usually ranks in the top ten in terms of quality of life), Japan (population has been declining for some time), South Africa (because of their biodiversity loss, Skowno et al. 2021), and India (supplies most immigrants to Canada). In the following table, sourced from many individual country and world population web pages, population is in millions, and 2022LE denotes life expectancy in 2022. Plotted is the ratio of the country’s population to that in 1880.

Table 1 Statistics from various sources on the web, population in millions, 2022 LE denotes life expectancy in 2022.

	1880		1910	1940	1970	2000	2030	2050	2100	2022LE
SAfrica		3.14	5.77	11	22.1	45	66	73.53	79.19	62.8
Canada		4.36	7.21	17.9	21.4	30.6	40.8	45.89	56.95	82.42
Japan		35.93	51	73.1	105	128	121	103.8	75	86.74
India		223	254	324	555	1057	1515	1670	1090	70.19
Sweden		4.6	5.52	6.37	8.04	8.87	11	11.9	13.02	82.18
World		1400	1777	2324	3682	6114	8600	9709	10400	72.98

Figure 1 Population growth relative to 1880, observed and projected.

Note the significantly different population trajectories of Sweden, on the one hand, and of South Africa and Canada on the other (all start with similar populations), and between South Africa and Japan (who end with comparable populations). South Africa has by far the largest growth rate (25X) and by far the lowest life expectancy, where civil life has all but broken down (Wikipedia 2023a) Japan has the lowest growth rate (2.1X) and highest life expectancy. These numbers clearly support the contentions of the experts cited above in this paper, that Canada will run out of immigrants (at least from India), that degrowth leads to higher quality of life, and that rapid growth leads to a decline in quality of life, conclusions which seem to be at odds with current Canadian immigration policy.

We conclude that degrowth is a serious issue that ought to be considered in Water Systems Engineering, especially in planning, design, construction, management, and rehabilitation of water systems.

4 Planning, designing, and building for ultimate degrowth

Evidence everywhere points to existing, imminent, and eventual degrowth, already or definitely within a few generations in developed countries, and certainly in a shorter timespan than it will take to harmlessly weather away our planned engineered modifications such as vegetal habitats (estimated to be many millenia, if ever), landscape interventions (varies, up to several millenia), concrete (a millenium), and steel construction (centuries) [cf. Weisman 2007].

Dasgupta (2021) prepared a report for the British government about the financial value of nature which found that "biodiversity has not only instrumental value, it also has existence and intrinsic value, perhaps even moral worth. Each of these senses is enriched when we recognise that we are embedded in Nature. To detach Nature from economic reasoning is to imply that we consider ourselves to be external to Nature." The report does not contemplate population decline and is a convincing argument for restoring Nature and rewilding modified areas.

Hence, water resources modelers might consider at the outset how their works, suffering, as they are bound to, eventual demand shrinkage, may be returned to the earlier prevailing environment as benignly as possible, bearing in mind that people may locally continue to “flee” remote rural villages, “flocking” to cities (or not). Property in certain areas will likely become more affordable, while labour might become locally scarce. To reduce financial disruption, ownership of land in developments planned for restoration to “managed wilderness” might be leased rather than sold freehold (bear in mind however that married, single children inherit wealth, land, housing, and assets from both sets of parents, an effect which increases exponentially with each passing single-child generation).

5 Purpose of water management models restated

*Water management modelers need no reminding that civil life begins (and ends) with water management: viz. the isolation of wastewater from potable water, protection from rain, floods, and droughts, and provision of that most ubiquitous of all engineered structures: pavement – all of which are basic elements of water models. [*This is the eleventh proposition, alluded to earlier.]

Simply stated, the purpose of the water management models envisaged here is to compute the many time series of water and pollutant flux at all points in a given catchment, with all of its almost unimaginable water facilities and fixtures, resulting from a representative time series of causative weather. More specifically, the time-distribution of computed water depths and water velocities allow inferences to be drawn regarding flood damages, river morphology, and aquatic ecology. From an engineering standpoint, stormwater models estimate the impact of engineered modifications to the hydraulic and hydrologic environment, such as stormwater ponds, rain gardens, swales, and artificial wetlands, and permit their planning, detailed design, and construction and management. Thus, for various time horizons and for various scenarios, cost-effective, manageable water drainage systems can be planned and simulated, a significant capability, given the cost of constructing, maintaining, repairing, and removing such systems.

Next, we consider long-lived water models that are a critical part of communities’ long- and short-term planning and development, for both growth and degrowth conditions.

6 Five suggestions for longer-term water systems modeling

First, that modelers in future consider (plan, design, and model) four key scenarios, two more than conventional: new (i) ahead, and (iv) after the conventional two: (ii) the as-is, (iii) and to-be; all four should be considered a matter of professional ethics towards the long-term preservation of the environment:

1. As-was: (New suggestion) The presumed landscape before the Anthropocene (e.g., in forested North America, a landscape whose hydrology was in many northern places controlled by the fascinating keystone species, the North American beaver, Castor canadensis) (Wikipedia 2023b).
2. As-is: The landscape existing at the time of modeling, which models can be properly calibrated.
3. To-be: The desired landscape to be developed for the short to medium term. This landscape could feasibly be planned for demolition when the population declines.
4. Reversion: (New suggestion) A presumed distant future (post population decline) landscape vaguely similar to both the As-was and To-be scenarios, but much altered: partially restored and partially invaded by alien and exotic vegetation, subject to deteriorating drainage systems, collapsing stormwater storages and blocked sewers, severely modified soils, and weathered and decayed concrete pavement, plausibly including (where appropriate) fostering keynote species and their habitat. Currently popular urban drainage models would obviously have to be elaborated to include code for beaver-dominated hydrology and hydraulics.

Second, since Crawford‘s water systems models run at a fine time and spatial resolution over millennia of simulated time (Crawford 2022), catchment development can now be simulated and visualised, morphing between the above four key scenarios. In this way, the public can time-zoom into the simulation to understand their local landscape changes over the various periods of their own family memory, e.g., in the writer’s case 1880 till 2100. By publicly reporting accumulated increased total runoff pollutional loads, the public is made more responsible for their water environment impacts. Civic leaders, on the other hand, zoom into the complete time horizon, from original old-growth temperate rainforests of e.g., tall sugar maple, beech, and hemlock, thickly carpeted with moss and lichen, to the reversion landscape overrun by invasive, alien vegetation of e.g., ailanthus, Russian olive, Norway maple, buckthorn, black locust, phragmites, and the residue of manmade interventions and drainages. Simulated time would cover 2–4 millennia and the computations and their results shared widely in social media. At present (2023) communities do not broadcast estimates of accumulated total pollutional loads.

Third, the hydrology of restored urban landscapes with, where appropriate, keystone species, and the hydraulics of their artificially instigated impoundments, if any, must be implemented in water systems models, including such issues as the adequate re-colonisation of floodway fringes by preferred feedstock, e.g., willow, maple, poplar, beech, birch, alder, aspen, and aquatic vegetation. Beaver hydrology (sequence of impoundments and narrow canals) in particular, if re-established, is likely to be the antithesis of existing urban areas (impervious pavement, roofing, sewers, and traditional stormwater infrastructure). Very probably, suitably modified existing code coupled with information from cognate disciplines, will suffice.

Fourth, phased reversion of an urban to a sustainable conservation area should progress in a planned direction, perhaps linking headwaters of sub-catchments along catchment divides, to provide unpolluted runoff and continuous green forest for the restored wilderness. Contraction of human living spaces, planned obsolescence of drainage, and planned restoration of wilderness may constitute novel applications for water systems modelers.

Fifth, as and if required, carry out field research to: (i) Clarify the hydrological implications of principles of wildlife and habitat restoration for the best sequence of systematic degrowth and re-wilding of former urban areas (Passmore 2019). (ii) Determine suitable empirical relationships, coefficients, and constants for the hydrology of adapted former urban environments. (iii) Develop economic tools for leasing land for occupation now, and later reversion to conservation areas.

7 Reasonable weather generators are key

Steinschneider et al. (2019) at Cornell University, developed a stochastic weather generator (precipitation, temperature, solar radiation, and wind) that has to be trained or adapted to local historic data series. Crawford found that the generated meteorological data series are reasonable compared to observed data series, but do not necessarily create similar high flow or drought behaviour in water models. The authors claim that their generators adequately preserve many of the historical statistics for precipitation and temperature across sites, including the mean, variance, skew, and extreme values. Cornell’s weather generator is “semiparametric by design, and includes (1) a nonhomogeneous Markov chain for weather regime simulation; (2) block bootstrapping and a Gaussian copula for multivariate, multisite weather simulation; and (3) modules to impose thermodynamic and dynamical climate change, including Clausius‐Clapeyron precipitation scaling, elevation‐dependent warming, and shifting dynamics of the El Niño–Southern Oscillation (ENSO)”.

Such models allow long-term investigations of alternate interventions.

8 Finally, a sting in the tail

Reactionary pro-growth positions are rife in the media, promising abundance for all, even though for most people an extension of the current situation holds little appeal. Yet, depopulation in various places and at various times is inevitable and will eventually be widespread. Consequent degrowth implications are important, including phased contraction of living space and restoration of “wild” areas. Long-term water management modeling directed towards this issue is yet to be reported in the literature. Conservative engineering practice, encumbered as it is by codes, standards, and the process of officially accepting water models, will be a brake and attenuate change, so engineering leadership may be called for. When human populations decline to earlier levels, humanity will inevitably discover an improved quality of life.

But, in that brave new world there remains the risk that humankind’s dark side might eventually return us to our present self-destructive culture—in this respect the last four stanzas of Rudyard Kipling’s century-old poem “The Gods of Copybook Headings” (Kipling 1919) are preternaturally apt—innate forces compel us, as in the bible stories of a dog returning to its barf, of a pig to its muck, or of re-testing heat with an already burned finger.  

In the Carboniferous Epoch we were promised abundance for all,
By robbing selected Peter to pay for collective Paul;
But, though we had plenty of money, there was nothing our money could buy,
And the Gods of the Copybook Headings said: "If you don't work you die."  

Then the Gods of the Market tumbled, and their smooth-tongued wizards withdrew
And the hearts of the meanest were humbled and began to believe it was true
That All is not Gold that Glitters, and Two and Two make Four
And the Gods of the Copybook Headings limped up to explain it once more.

As it will be in the future, it was at the birth of Man
There are only four things certain since Social Progress began.
That the Dog returns to his Vomit and the Sow returns to her Mire,
And the burnt Fool's bandaged finger goes wabbling back to the Fire;

And that after this is accomplished, and the brave new world begins
When all men are paid for existing and no man must pay for his sins,
As surely as Water will wet us, as surely as Fire will burn,
The Gods of the Copybook Headings with terror and slaughter return!

[Present writer’s interpretation: The poet warns that even though water management models might help phase a transition to prosperity without growth, the models do not protect against a possible eventual return to the values of our darker (acquisitive) side…]

References

1. Bloomberg News. 2023. “The countries with the longest and shortest retirements.” https://www.bloomberg.com/graphics/2023-global-retirement-age/?leadSource=uverify%20wall. (Accessed 2023.03.21).
   Data, graphics and development by Jane Pong, Yasufumi Saito and Adrian Leung. Edited by Emily Cadman, Ainsley Thomson, Jin Wu and Spe Chen. Sources: WHO, OECD, data compiled by Bloomberg. Methodology: To calculate length of retirement period, Bloomberg News obtained life expectancy and healthy life expectancy data from WHO, and effective labor market exit age data from OECD. These were then combined for all 38 OECD countries. As the focus was on the retirement experience, data for life expectancy and healthy life expectancy are for 60-y-olds, those who die earlier are excluded. The healthy period after retirement is defined by subtracting effective retirement age from healthy life expectancy. The dataset covers 2000, 2010, 2015 and 2019. The country comparison graphics use the 2019 data.
2. Cody, J. 2022. "Swiss national referendum will limit population to 10 million through strict immigration control to save environment." Remix. https://rmx.news/article/swiss-national-referendum-will-limit-population-to-10-million-through-strict-immigration-control-to-save-environment/.
3. Crawford, N. 2022. Personal communication. CHI Zoom meeting of original water model code developers. 2022.12.15.
4. Dasgupta, A., and P. Dasgupta. 2017. “Socially Embedded Preferences, Environmental Externalities, and Reproductive Rights.” Population and Development Review 43: 3, 405-441. https://doi.org/10.1111/padr.12090​. (Accessed 2023.03.21).
5. Dasgupta, P. 2021. The Economics of Biodiversity: The Dasgupta Review. (London: HM Treasury) Available at: https://www.gov.uk/government/publications/final-report-the-economics-of-biodiversity-the-dasgupta-review.
6. Feng, W. 2023. “The Alternative, Optimistic Story of Population Decline.” Guest essay, New York Times. https://www.nytimes.com/2023/01/30/opinion/china-world-population-decline.html. (Accessed 2023.03.21).
7. Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES). 2019. “Global Assessment Report on Biodiversity and Ecosystem Services.” Brondízio, E.S., J. Settele, S. Díaz, H.T. (eds). IPBES secretariat, Bonn, Germany. 1144 pages. ISBN: 978-3-947851-20-1. https://www.ipbes.net/global-assessment. (Accessed 2023.03.21).
8. Jackson, T. 2009 “Prosperity without growth: Economics for a finite planet.” Earthscan, London, 284pp.
9. James, W. 2023. “Metamorphosing the pachyderm in the room into Panacea and her siblings: how water systems models can help preserve civil life.” Keynote paper, 56th International Conf. on Water Management Modeling. https://www.icwmm.org/Archive/2023-C032-01/keynote-metamorphosing-the-pachyderm-in-the-room-into-panacea-and-her-siblings-how-water-systems-mod. (Accessed 2023.03.21).
10. James, W. 1971. “Civil Engineers and ‘Populution." Transactions of South African Institution of Civil Engineers 13, 9 (Oct), 365-368.
11. Kipling, R. 1919. “The Gods of the Copybook Headings” (first published in the Sunday Pictorial of London on 26 October 1919, in America as "The Gods of the Copybook Maxims" in Harper's Magazine in January 1920). The Poems in The Kipling Society. https://www.kiplingsociety.co.uk/poem/poems_copybook.htm. (Accessed 2023.03.21).
12. Kolinjivadi, V. 2022. “The coronavirus outbreak is part of the climate change crisis.” https://www.aljazeera.com/opinions/2020/3/30/the-coronavirus-outbreak-is-part-of-the-climate-change-crisis/. (Accessed 2023.03.21).
13. Maynard, R., and M. Scigliano. 2022. “The BBC on Population: re-treading a well-worn path.” Population Matters, 18th January 2022. https://populationmatters.org/news/2022/01/the-bbc-on-population-re-treading-a-well-worn-path/.
14. Meadows, D.H., D.L. Meadows, J. Randers, and W.W. Behrens III. 1972. “The limits to growth.” Potomac Associates – Universe Books.
15. Murányi, A., and B. Varga. 2021. “Relationship Between the COVID-19 Pandemic and Ecological, Economic, and Social Conditions.” Front Public Health 9: 694191. eCollection 2021.  https://doi.org/10.3389/fpubh.2021.694191. (Accessed 2023.03.21). 
16. O’Neill, D.W., A.L. Fanning, W.F. Lamb, and J.K. Steinberger. 2018. “A good life for all within planetary boundaries.” Nature Sustainability 1, 88–95. https://www.nature.com/articles/s41893-018-0021-4. (Accessed 2023.03.21).
17. Our World in Data. 2023. https://ourworldindata.org/grapher/gdp-per-capita-worldbank?tab=table. (Accessed 2023.03.21).
18. Passmore, S. 2019. “Landscape with Beavers.” Places Journal, July 2019. https://doi.org/10.22269/190709.
19. Population Matters. 2021a. “Smaller Families and Ageing Populations, Silver linings not silver burdens.” A Population Matters White Paper. https://populationmatters.org/resources/silver-linings-not-silver-burdens-smaller-families-and-ageing-populations-white-paper/. (Accessed 2023.03.21).
20. Population Matters. 2021b. “Welcome to Gilead: How Population Fears Drive Women’s Rights Abuses.” https://populationmatters.org/news/2021/11/welcome-to-gilead-how-population-fears-drive-womens-rights-abuses/. (Accessed 2023.03.21).
21. Reynolds, M. 2022. “Elon Musk Is Totally Wrong About Population Collapse.” Wired. Oct 6, 2022. https://www.wired.com/story/elon-musk-population-crisis/.
22. Ripple, W.J., C. Wolf, T.M. Newsome, P. Barnard, and W.R. Moomaw. 2019. "World Scientists' Warning of a Climate Emergency." BioScience 70 (1): 8–12. https://doi.org/10.1093/biosci/biz088.
23. Skowno, A.L., D. Jewitt, and J.A. Slingsby. 2021. “Rates and patterns of habitat loss across South Africa's vegetation biomes.” South African Journal of Science 117 (1-2), 1-5. https://doi.org/10.17159/sajs.2021/8182​. (Accessed 2023.03.21).
24. Steinschneider, S., P. Ray, S. Haque Rahat, and J. Kucharski. 2019. “A Weather-Regime-Based Stochastic Weather Generator for Climate Vulnerability Assessments of Water Systems in the Western United States.” Water Resources Research 55 (8): 6923-6945. https://doi.org/10.1029/2018WR024446. (Accessed 2023.03.21). 
25. Tremblay, R. 2022. “Canadian Immigration Policy and the Toronto Lobby's 'Century Initiative' Project”. https://rodriguetremblay100.blogspot.com/2022/06/. (Accessed 2023.03.21).
26. Weisman, A. 2007. “The World Without Us.” St Martin’s Thomas Dunne Books, ISBN 978-0-312-34729-1. (Accessed 2023.03.21).
27. Wikipedia. 2022. "Degrowth." Wikimedia Foundation. Last modified September 2023. https://en.wikipedia.org/wiki/Degrowth. (Accessed 2023.03.23). 
28. Wikipedia. 2023a. “2021 South African Unrest.” Wikimedia Foundation. Last modified 2023.08.12. https://en.wikipedia.org/wiki/2021_South_African_unrest. (Accessed 2023.03.21).
29. Wikipedia. 2023b. "North American Beaver." Wikimedia Foundation. Last modified 23 September 2023. https://en.wikipedia.org/wiki/North_American_beaver. 
30. Wikipedia. 2023c. “Population decline – Possible negative effects.” Last modified 2023.10.16. https://en.wikipedia.org/wiki/Population_decline#:~:text=Other%20possible%20negative%20impacts%20of,are%20insufficient%20caregivers%20for%20them. (Accessed 2023.03.21).
31. Wikipedia. 2023d. "Quality of Life." Wikimedia Foundation. Last modified 12 October 2023. https://en.wikipedia.org/wiki/Quality_of_life. (Accessed 2023.03.23).
32. Wikipedia. 2023e. "Resilience." Wikimedia Foundation. Last modified 24 September 2023. https://en.wikipedia.org/wiki/Resilience. (Accessed 2023.03.23).
33. Wikipedia. 2023f. “World Scientists' Warning to Humanity.” Wikimedia Foundation. Last modified 7 October 2023.  https://en.wikipedia.org/wiki/World_Scientists%27_Warning_to_Humanity#CITEREFRipple2019. (Accessed 2023.03.21). 
    The "World Scientists' Warning to Humanity" was a document written in 1992 by Henry W. Kendall and signed by about 1,700 leading scientists. Twenty-five years later, in November 2017, 15,364 scientists signed "World Scientists' Warning to Humanity: A Second Notice" written by William J. Ripple and seven co-authors calling for, among other things, human population planning, and drastically diminishing per capita consumption of fossil fuels, meat, and other resources. The second notice has more scientist cosigners and formal supporters than any other journal article ever published.
34. World Commission on Environment and Development. 1987. “Our Common Future." 300pp. https://sustainabledevelopment.un.org/content/documents/5987our-common-future.pdf. (Accessed 2023.03.21).
35. World Health Organisation. 2023. https://www.who.int/activities/promoting-well-being. (Accessed 2023.03.23).

APPENDIX: Questions raised at the live presentation

An earlier version of this paper was presented live at the 2023 International Conference on Water Management Modeling (James 2023); during the short question period, the following points were raised and answered:

1. This is a difficult message for some of us—and we modelers and engineers are often the messengers to regulators and elected officials. Do you have any experience with how politicians, lawyers, and regulatory leaders respond to this message?”
   Answer: My first paper and presentation on population and well-being was in 1971 (James 1971) and it went hard, but it has become easier with each passing decade. Successes of our political Green Party have influenced attitudes, and today politicians and regulators are incomparably more respectful of degrowth. Globally, leaders of New Zealand, Finland, Iceland, and Scotland, among others, for instance, have also helped. The difficult message is evidently becoming mainstream, so don’t be caught out.
    
2. Degrowth could also be drastically impacted by climate change, which could transfer population as climate refugees from currently high population sections of the world (South Asia) to developed portions (North America and Europe).”
   Response: It seems certain that degrowth will slow or even halt further climate change; evolving climate models will elucidate whether degrowth can actually reverse global warming.
    
3. “How should we account for degrowth in infrastructure design?”
   Answer: Favour: indigenous plantings and endemic species; embankments and grass swales rather than concrete; permeable pavement, not asphalt of any kind; daylight drains rather than buried conduits; surfaces and drainages that restore infiltration and replicate historic conditions; but above all, (the point of this paper), use water models that account for phased obsolescence of engineered landscapes. – Just a few ideas, of course.
    
4. “Recently, in Colombia, the minister of Mines and Energy mentioned the theory of degrowth in a public panel and the media mocked her for proposing that a developing country slows down their growth under the expectation that developed countries will do the same. How would you recommend promoting this theory/concept in developing countries?”
   Answer: Emphasize that single-child families accumulate the assets of both sets of their parents, and similarly will enjoy exponentially increasing space and housing, without any loss of prosperity. Wages, benefits and working conditions will improve, prices will decrease, the environment and well-being will improve.
    
5. “Wouldn't Japan have a high life expectancy even with a higher growth?”
   Answer: Possibly, I don’t know when constraining limits will be reached. However, well-being increases with population decline, and life expectancy usually increases with well-being.
    
6. “Do you have any insights on learning to "slow down" PRE retirement?”
   Answer: Everyone is different, and there is endless advice on the web of course, though the main components are well established (healthy exercise and life style, companionship, volunteering, reading and writing, fun hobbies, etc.)
    
7. “One of your claims is that degrowth (specifically population degrowth) leads to improved quality of life. Most claims I've seen say that increased quality of life lead to reduced birth rates, not decreased population growth leading to increased quality of life (which is often used to justify increased development). Is there evidence showing that the degrowth is the cause and the improved quality the effect?”
   Answer: The drop in fertility is attributed to many causes, and they vary over history and from place to place, and especially from study to study. (Accepting that it is one of several causes, empowerment of women through education, along with education of both partners, is my preferred approach, because it is overdue and readily achieved.) Japan and Germany, for example, have shown that quality of life improved with population decline (Wikipedia 2023d). On the other hand, the USA, for instance, shows that population growth has no positive effect on quality of life.