Monsoon Harvests: The Living Legacies of Rainwater Harvesting

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Monsoon Harvests: The Living Legacies of Rainwater Harvesting Systems in South India Kimberly J. Van Meter,† Nandita B. Basu,*,†,‡ Eric Tate,§ and Joseph Wyckoff§ †

University of Waterloo, Department of Earth & Environmental Sciences, Waterloo, Ontario N2L 3G1, Canada University of Waterloo, Department of Civil Engineering, Waterloo, Ontario N2L 3G1, Canada § University of Iowa, Department of Geography, Iowa City, Iowa 52242, United States of the 20th century,8 it is currently experiencing a revival. Organizations ranging from small nongovernmental organizations (NGOs) to the World Bank are now heavily investing in the renovation of these small-scale water-storage structures in the pursuit of increased water availability and more sustainable livelihoods.9,10 RWH in India is estimated to have the potential to add as much as 125 km3/year to the current water supply, making it central to many plans to meet the country’s projected midcentury water shortfall of over 300 km3/year.11 Indeed, in 2005 India revealed its Groundwater Recharge Master Plan, which called for the renovation or new construction of a variety of RWH structures, including the village tanks of South India, at a cost of approximately $6 billion.12 Rainwater harvesting, a “soft path” approach toward water Many researchers and development professionals now management, is increasingly recognized as a key strategy toward suggest that small-scale water solutions are the most costensuring food security and alleviating problems of water effective, efficient, and environmentally neutral means of scarcity. Interestingly this “modern” approach has been in use meeting demand in water-stressed regions.13−15 These for millennia in numerous older civilizations. This article uses solutions fall under the rubric of the "soft path" approach to India as a case study to explore the social, economic, and water management, in which existing large-scale projects are environmental dimensions of agricultural rainwater harvesting complemented by small-scale, decentralized solutions.16 Iniponds, and evaluates the viability of these centuries-old systems tiatives including such soft-path approaches extend beyond under current climate and population pressures. A holistic India, with the revival of traditional RWH systems from China watershed-scale approach that accounts for trade-offs in water to the Middle East,17,18 and the transfer of RWH technologies availability and socioeconomic wellbeing is recommended for to water-stressed areas such as Sub-Saharan Africa with no longassessing the sustainability of these systems. term history of RWH.19 Numerous international forums have 1. INTRODUCTION identified RWH as an integral component in interventions necessary to meet Millennium Development Goals,20 and there Lack of consistent water availability for irrigated agriculture is has been widespread adoption of RWH systems for now recognized as one of the primary constraints to meeting supplemental irrigation in semiarid areas of Kenya, Ethiopia, UN Millennium Development Goals to alleviate hunger, and and Ghana, to name only a few.19,21 Even in the U.S. there has multiple studies indicate that by 2025 all countries will face 1,2 been growing interest in the use of RWH for a range of some form of water stress. Water shortages promise to purposes, from reducing stormwater runoff and preventing become more acute as population pressures increase, with watershed pollution,22 to augmenting water supply during current projections indicating a 21% increase in global water drought years.23 consumption for grain production by 2050.3,4 Particularly in Although stories abound in India of successful renovations of semiarid landscapes with high seasonal rainfall variability, RWH ponds, with whole villages reportedly being revitalized significant correlations have been found between a lack of after years of extreme water stress, the extent to which this surface water storage, reduced food security and poor economic 5−7 centuries-old technology can fundamentally address problems development. of water scarcity has been called into question.9 The To meet the demand for seasonal water storage, village-level assumption of those funding the renovation seems to be that rainwater harvesting (RWH) systems have been in use in India social and economic benefits will accrue at the village scale as for millennia. In the agricultural areas of South India, where we overall water availability increases.24 But can RWH truly focus our study, these structures have commonly taken the increase water availability at the basin scale? In the face of form of large (20−40 ha) earthen impoundments, referred to as climate change and population pressures, does utilization of “tanks,” that can collect water during the monsoon season, these ancient technologies represent the best path toward making surface water stores available to farmers for irrigation. solving problems of water scarcity? Although the use of RWH systems began to decline in India with the building of large-scale irrigation structures under British colonial rule and with the groundwater pumping boom Published: February 27, 2014 ‡

© 2014 American Chemical Society

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dx.doi.org/10.1021/es4040182 | Environ. Sci. Technol. 2014, 48, 4217−4225

Environmental Science & Technology

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Figure 1. Components of rainwater harvesting systems in Tamil Nadu: (A) tank sluice at low water levels; (B) irrigated fields in tank command area; (C) open well in tank command area with electric pump set; (D) community well next to large tank at high water levels; (E) statues of Hindu deities on tank bund, protecting the tank; (F) goats grazing near dead storage area of tank; (H) tank systems, as seen via the remote sensing image, are ubiquitous throughout the state.

of the tanks, however, is considered to extend well beyond that of a water source for agriculture. They are essentially humanbuilt ecosystems, providing economic, socio-cultural, and ecological services to their communities. Ecologically, these tank systems make up an extensive, interconnected web of manmade wetlands providing a broad range of ecosystem services, including flood control, nutrient and waste removal, provision of avian habitat, and increased biodiversity of flora and fauna. In Tamil Nadu alone, there are more than 39 000 tanks supporting wetland biodiversity and adding significantly to the country’s wetland wealth.26 Tanks also enhance groundwater recharge and increase stream baseflow,27 helping to revive rivers that have been depleted by large-scale diversions for canal irrigation and other uses. Historically, tanks were central to settlement patterns in South India: settlements grew up around temples, and over time temples and tanks became nearly inseparable.28 In villages, temples are often built directly into tank bunds, with statues of the deities positioned nearby to protect the tanks from harm (Figure 1e). The tanks also provide social gathering places29 and in many cases are considered sacred spaces, with traditional rituals such as Hindu weddings and burial ceremonies taking place there.30 In daily village life, tanks continue to have multiple uses, providing water for drinking and the laundering of clothes, a grazing area for livestock (Figure 1f), trees for fuel wood, and silt to be used as a fertilizer.31,32 The varied uses of the tanks serve to strengthen the economic base of surrounding communities,33 with per capita incomes in some areas increasing more than 50% after tank rehabilitation projects.34 Conversely, tank decline has been found to result in reduced incomes, with these reductions being borne disproportionately by marginalized members of the community.35

Our objective herein is to explore the extent to which ancient RWH practices are applicable in the context of modern socioeconomic and environmental pressures. We use the South Indian state of Tamil Nadu, with its long history of RWH, as an example to address broader issues regarding the sustainability of RWH practices, not just in rapidly developing India, but also in other water-scarce areas of the world. In particular, we explore the ways in which RWH may impact basin-level water stores and fluxes, and how these impacts may contribute to shaping the socioeconomic landscape within a closely coupled human and natural system.

2. SOUTH INDIA’S RWH SYSTEMS: STRUCTURE AND FUNCTION The RWH “tanks” of South India are formed via the construction of earthen banks, or bunds, across natural depressions in the landscape to impound surface runoff (Figures 1 and 2). During monsoon rains, runoff from the tank catchment area inundates the tank bed. Sluices are constructed in the tank bund, each with a shutter that can be controlled to manage the outflow of water from the tank into irrigation channels (Figure 1a) that route water to downstream agricultural fields (Figure 1b), where wells may also be present to supplement tank irrigation (Figure 1c, d). Tanks are often linked in cascades (Figure 1g, Figure 2), with overflow from upstream tanks spilling over into surplus channels leading to downstream tanks or nearby waterways. The tank cascades, which can encompass anywhere from several to more than a hundred tanks, create a hydrological network across the landscape, providing points of connection not just between individual tanks, but also small farm ponds, wells, and rivers. The tank irrigation systems of Tamil Nadu support an agricultural area covering 61% of the state,25 and allow for the growth of subsistence crops like rice, as well as market crops such as maize, sugar cane, and chili peppers. The functionality 4218



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Figure 2. Major elements of a typical tank irrigation system are shown, note the horseshoe shape of the bund and the water pooling behind it.

3. LIVING LEGACIES: HISTORY AND CURRENT STATE OF RAINWATER HARVESTING SYSTEMS IN TAMIL NADU The ancient RWH structures of South India have persisted, essentially unmodified, into the present day, making them living legacies of early farmers’ triumphs over the harsh, semiarid environment. Evidence of tank irrigation in Tamil Nadu dates back to the Sangam period of 150 BC to 200 AD,36 and by the early medieval period (750−1300), tank irrigation was thriving throughout the region. Archaeological and historical records indicate a correlation between periods of prolonged drought and developments in RWH practices.37 For example, after a period of many severe monsoon failures from AD 505−550 was written the Brihat Samhita, an encyclopedic work providing extensive advice regarding the construction of tanks. The historic reliance on tank irrigation systems in Tamil Nadu began to wane during India’s colonial era (1757−1947), and then declined even further in the late 1960s with the advent of the Green Revolution.38 The British focused on large-scale irrigation projects such as the construction of large dams and canal networks, often at the expense of village-based tank systems.8 The Green Revolution, accompanied by the increased

availability of diesel and electric pumpset technology and rural electrification,33,39 led to dramatic increases in the number of irrigation wells in Tamil Nadu, from an estimated 50 000 in 1905 to a documented 229 394 in 1971.40 The increased access to irrigation water has been a boon to economic development and agricultural productivity in India, with yields 1.2−3 times greater in areas irrigated by groundwater.41 However, increased groundwater use has also led to alarming levels of groundwater depletion (Figure 3a).42,43 In Tamil Nadu, recent estimates by the government Central Ground Water Board suggest that more than a third of Tamil Nadu’s groundwater resources are overexploited,44 with the annual groundwater draft exceeding the mean annual recharge (Figure 3b). Of the estimated 1.8 million groundwater wells in Tamil Nadu, approximately 12% are dried up or abandoned due to overexploitation, and in some areas well failure rates are greater than 40%.45,46 In response to these changes, the area irrigated by tanks has continued to decline, from an estimated 900 000 ha down to 500 000 ha (Figure 3c) over the last 40 years.47 The increased reliance on groundwater has led to a frequent neglect of traditional water governance organizations, and thus to declines in tank maintenance. Many tanks have succumbed to structural failures and the encroachment of fields into tank beds,33 leading 4219



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Figure 3. Groundwater withdrawals are shown as a percentage of recharge (a) across India (based on state-level estimates from the Indian Ministry of Water Resources)43 and (b) in the Indian state of Tamil Nadu (based on (district-level estimates from the Tamil Nadu Central Ground Water Board).44 Figure 1 (c) shows the percentages of land irrigated by tanks and wells in relation to the total irrigated area,3,4 with canals and rivers accounting as additional irrigation sources. Note that the high levels of depletion within the state are a result of the expansion of well irrigation, at the expense of traditional tank systems over the last 50 years.

leaving the sustainability of the village in doubt.49 With shortterm migration being a common adaptive response to water scarcity, long-term shifts in water availability due to groundwater depletion can lead to a decline in economic prospects and growing social instability at a regional scale.50

to even less community-level investment in the tank systems and an overall devaluing of the commons resource. The decline in tank irrigation paired with virtually uncontrolled groundwater depletion has led to a complex series of negative environmental and socio-economic feedbacks, particularly for poor and marginal farmers, for whom loss of groundwater resources has been directly correlated to a loss in food security.42,48 While wealthier farmers may be able to afford the costs associated with ever-deeper wells, the poorest farmers, without access to groundwater and with decreasing availability of tank irrigation water, may be unable to grow rice, the region’s water-thirsty staple food crop. As a result, many may be forced to leave their land fallow and migrate to nearby urban centers,

4. RAINWATER HARVESTING AND THE WATER PORTRAIT In the 12th century, King Parakramabahu of Sri Lanka, who oversaw the building of a massive network of rainwater harvesting reservoirs, famously proclaimed “Let no drop of water flow to the sea unused by man.”51 Such a strategy seems to parallel the intensive RWH programs currently being 4220



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Figure 4. Representation of the coupled natural and human system of south Indian agriculture as a function of both natural and anthropogenic drivers is depicted. The figure demonstrates how the complex web of feedbacks within this system can lead to unintended consequences. Tank restoration increases food security, while concurrently reducing income disparities (since even the poorest farmer has access to tank water), and increasing social equity. From an environmental perspective, tank restoration also can increase local groundwater recharge and thus increase environmental flows. Restoration projects are therefore being heavily promoted in India as a sustainable alternative. Problematically, however, the increased recharge and rising groundwater levels often trigger increased pumping (represented as bidirectional arrows between pumping and GW levels), which coupled with increased surface water depletion by irrigation and greater evaporation losses, can ultimately lead to reductions in environmental flows.

regional flow standards. In India, with its semiarid climate and intensive use of water, rivers are in many areas dry in most years,9,53 and estuaries and mangrove wetlands along the coasts are becoming increasingly saline due to reduced freshwater inflows.54 Ideally, water management plans, including those involving RWH, should be designed to ensure that environmental flows to estuaries are maintained even in the driest years, and that only “excess” water is allocated for irrigation use. Next, the “excess” water must be allocated within a watershed, taking into account the many different trade-offs. Studies indicate that increases in the number of RWH systems leads to decreases in water availability in downstream reservoirs, and a consequent increase in upstream-downstream conflicts, especially in the drier years.19 Furthermore, in semiarid landscapes like South India, the high surface area-to volume ratio of the RWH tanks can lead to large evaporative losses, creating a pathway for net water depletion from the basin.55 Although environmental benefits can accrue from increased recharge, which increases groundwater levels and potentially sustains baseflows in streams over longer periods,52 whether or not recharge is a substantial benefit of the tanks is a function of

developed within India. The question remains, however, as to whether this approach indeed provides a sustainable alternative in the modern world. To answer this question it is necessary to understand how RWH alters the water portrait (stores and fluxes) within a basin, and how the water portrait shapes the social landscape. An obvious but perhaps overlooked fact of RWH is that it does not increase the overall water availability within a basin, as sometimes touted, but merely alters the distribution of water between upstream and downstream users, and between socioeconomic and environmental demands. The result is a trade-off of water availability, and any proper evaluation of a RWH system requires understanding, quantifying, and prioritizing these multiple uses based on local and regional needs. Fundamentally, any such evaluation must first consider the central trade-off between maintaining environmental water flows and harnessing these flows for human use. Recent work has framed this consideration of environmental flows in terms of what are referred to as the ecological limits of hydrologic alteration (ELOHA).52 Establishing the boundaries of such ecological limits is essential to establishing ecologically based 4221



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hydrology) are indirect and entrenched determinants of water sustainability, and are represented by the outer two rings. Pressures are the specific processes produced by combinations of natural and human system drivers, and occupy the next inner ring. The States and Impacts are collectively identified in the innermost ring, while Responses are societal actions intended to remedy impacts, and feed back to driving forces and pressures. As an example of these dynamics, Figure 4 focuses on the particular pressures of groundwater pumping and tank management. Excessive pumping influences the State of the CHANS attributes (e.g., increases in irrigated area and income, decreases in groundwater levels, and functionality arising from excessive reliance on groundwater extraction). Increased reliance on groundwater pumping also negatively impacts tank management practices, thus exerting additional pressure on the system state, and leading to a series of positive and negative Impacts on the system (e.g., increases in income from increases in irrigated area, well failure from excessive groundwater depletion, reduced environmental flows from reduction in groundwater levels and recharge, and social inequity, as groundwater wells can only be afforded by wealthier farmers).35 Finally, these Impacts lead to a series of Responses like the engagement of NGOs and government organization in tank rehabilitation to raise income levels of the poorest farmers, as well as to mitigate excessive groundwater depletion. This network of cause and effect, as represented by the arrows in Figure 4, provides merely one example of the complexity of the CHANS system, many other possible connections exist. While analyzing the entire realm of connections is beyond the scope of this feature article, it is necessary to acknowledge that a proper analysis of RWH systems requires consideration of these linkages and feedbacks. These feedbacks contribute to the existence of thresholds, the notion that systems may fail in an abrupt, nonlinear fashion, which is yet another attribute of CHANS systems. Thresholds are notoriously difficult to identify in advance because sudden collapse may be preceded by a long period of gradual degradation, reducing the perceived urgency to adapt.65 Steady annual increases in groundwater pumping, and transitions to water-intensive crops (Figure 4) may not be remarkable when considered individually, but operating concurrently these pressures reduce the buffering capacity of the system to external shocks. In the event of a prolonged drought, systematic well failure, or significant bund breach, the agricultural system of a village with a degraded tank is at much greater risk of collapse, with potentially serious long-term impacts to the livelihoods of people and the viability of the community.

the soil on which the tanks are constructed. RWH tanks constructed on clayey soils have been judged to be nearly 100% inefficient with regard to groundwater recharge, causing them to act simply as evaporation pans.56,57 Modeling studies suggest that the benefits of additional RWH structures at the watershed scale may be minimal,55,56 with one recent study predicting that increasing the number of RWH structures would decrease runoff within the basin by 60%, while increasing groundwater recharge by only 5%.58 This shift in the water balance appeared to be primarily due to the increase in evapotranspiration caused by increased irrigation and changes in land use. Notwithstanding these factors, decentralized solutions like rainwater harvesting do have benefits over traditional irrigation systems. First, rainwater is used at the location where it falls on the land, and thus the transmission losses associated with extensive canal network systems are minimized. Second, RWH systems use rainwater, which has faster time scales of replenishment than groundwater, and thus can be considered to be a more sustainable source. Furthermore, water stored in a RWH tank is commonly distributed equitably between all villagers, in contrast to groundwater irrigation systems where only the wealthier farmers have access to irrigation water.34 Finally, as discussed above, the socioeconomic benefits provided by tanks extend beyond a source of irrigation water, and economic returns from tanks have been found to more than double when considering multiple uses.33

5. RAINWATER HARVESTING AS A COUPLED HUMAN AND NATURAL SYSTEM One of the most striking attributes of these millennia-old RWH systems is the close coupling that exists between the natural and the human systems. Just as the climate and geomorphology of the region have shaped the creation of these ancient waterdelivery systems and the social practices that have grown up around them, the systems themselves have shaped the environment that we see today in South India, where rivers run dry for years at a time, and invasive tree species colonize the tank beds, tapping into reservoirs of shallow groundwater. Such a close coupling of the human and natural systems is intensified by the sheer longevity of these RWH tanksin some cases they can be said to have existed with greater constancy than some natural ecological communitiesmaking it nearly impossible to separate this network of distributed storage from the natural system in which it is embedded.59 Accordingly, any consideration of rainwater harvesting as a solution to current problems of groundwater depletion must include an understanding of interacting social, economic, and ecological processes. Tank systems are affected by an array of forces operating at different geographic and administrative scales, with time lags between cause and effect, feedbacks between effect and cause, failure thresholds, and connections among environmental and human components. These characteristics are emblematic of coupled human and natural systems (CHANS), an emerging interdisciplinary framework for analyzing the dynamics of complex systems.60,61 Figure 4 shows a conceptual model of the tank irrigation system that borrows from both the CHANS perspective and the Driving Force-Pressure-State-ImpactResponse (DPSIR) framework often applied to environmental assessment and sustainable development.62−64 The model is bounded by the human and natural driving forces that combine and interact to exert pressure on the tank system. The Driving Forces (e.g., population, well technology, climate, and

6. A WAY FORWARD: CAN RAINWATER HARVESTING ADDRESS PROBLEMS OF WATER SCARCITY? Globally, as water use continues to escalate, aquifers are depleted, and more than a third of the population is affected by water scarcity, maintaining a stable water supply is perhaps more important than ever. As RWH is being revived or newly adopted in many areas of the world to address the challenges of limited water availability,17,18,21,66,67 we must ask whether these ancient technologies represent a sustainable alternative under modern socioeconomic and environmental pressures. The answer is not simple, and requires understanding and quantifying the socioeconomic and environmental trade-offs associated with these systems. For example, in areas such as Sub-Saharan Africa, where there might be a relative abundance of unallocated water resources, but a lack of economic means to 4222



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ACKNOWLEDGMENTS This research is financially supported by the U.S. National Science Foundation (1211968), Dynamics of Coupled NaturalHuman Systems. We thank the DHAN Foundation for generously sharing their experiences, data, and hospitality.

mobilize these resources, the low-cost, community-level water storage provided by RWH can play a valuable role in increasing yields and improving food security for small and marginal farmers.19 In contrast, in basins for which no extra water resources are available to meet consumptive water demands, the creation of new RWH structures can do little more than change the spatial distribution of water, essentially providing a downstream-to-upstream water transfer.9 From a socioeconomic perspective, however, even in such “closed” basins, increased use of RWH systems can lead to a more equitable distribution of the water supply, encouraging use of a commons resource instead of groundwater sources, which are generally privately held.33 Accordingly, RWH must be considered as just one part of an integrated plan to both maximize water availability and to manage demand. A first step along this path would be an accurate accounting of basin-wide water use by analyzing use, depletion, and water productivity at the basin scale.68 The benefits and potential of community-scale water management options must then be considered in the context of existing infrastructure as well as planned large-scale interventions. If a revival of RWH is deemed beneficial, it must be carried out concurrently with measures to manage demand, including the elimination of electrical subsidies for groundwater pumping, improvements in irrigation efficiency through drip irrigation, reductions in nonbeneficial evaporation from soil or supply sources, a shift to less water-intensive crops, or irrigating current crops at a deficit.27 Based on current trends, it is likely that RWH systems, both old and new, will play a role in addressing the challenges of limited water availability.12,20 Determining the appropriate bounds of this role, however, will require us to broaden our focus beyond the village scale, where the more tangible social and economic benefits of RWH may be seen, to the basin scale, where most environmental impacts and larger upstreamdownstream conflicts are felt. Currently, however, there are limitations in our ability to take such broad considerations into account. Data is lacking to support modeling efforts designed to clarify the regional hydrological impacts of RWH, and further research is necessary to determine optimal numbers of RWH structures within a watershed, rather than relying simply on historical precedence.47,55 It is also necessary to look beyond the water balance, and frameworks of analysis must be developed to place consideration of RWH solutions within the context of a coupled human and natural systems approach. One of the central tenets of the emerging science of sociohydrology is to account for the dynamics of interactions between water and people.69 In the spirit of such an endeavor, only a systems approach, accounting for the coevolutionary dynamics of the coupled human-water system, can allow us a full understanding of the potential for RWH to alter the social and hydrological landscapes of water-stressed areas throughout the world

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AUTHOR INFORMATION

Corresponding Author

*Phone: 519-888-4567, ×32257, 37917; e-mail: nandita.basu@ uwaterloo.ca. Notes

The authors declare no competing financial interest. 4223



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