Prof Satish Kailash
Prof Monto Mani
|Indian Institute of Science, Bangalore|
Many studies have warned that the alarming depletion of water table in many parts of India, unless urgently and effectively arrested, would be irreversible. This steep decline is traced to the over-exploitation of water for human consumption and agriculture. This paper postulates that the culprit behind the dramatic fall in water table can be attributed to one particular change in the farming methods – the use of chemical fertilizers. The use of chemical fertilizers has “opened” a traditional “closed cycle”. When one moves from a closed-cycle to an open-cycle, the system becomes unstable, unpredictable and unsustainable. While the focus of the paper is to evaluate the sustainability of ground water in the context of chemical fertilizer use, it is important to note that changing farming practices have further contributed to severe decrease in soil fertility and crop (genetic) diversity, increased energy consumption and consequent GHG emissions, excessive dissolved salts in water, and a general decline in robustness of the agricultural system to face climate-change vulnerabilities. The paper attempts to revalidate the ability of a closed-cycle system to support sustainable water resources and, possibly even, agriculture.
Water is fundamental to survival and development. While the total quantity of fresh water available might be adequate for current global population (demands) its distribution, accessibility and availability is inconsistent. The world’s most populous countries, India and China have per-capita water access of less than 500 m3/year and 1700 m3/year of respectively. These regions are already water stressed with projected figures indicating a 40+% stress by 2025. Stresses beyond this can have an irreversible cascading effect resulting in famine and starvation. Irrigation accounts for most of the water consumption (85%). The current domestic demands of 8% are the lowest in global comparison, with nearly 230 million inhabitants in south-Asia having no access to improved drinking water sources (WBCSD, 2006). Increasing rural-urban transitions, lifestyle changes and population growth will further increase domestic and agriculture water demands. It is evident that further change in irrigation water-demands will severely affect domestic water availability and sustainability of the region (Frank van Steenbergen and Tuinhof 2009). Irrigation water demands are determined by the farming practices. The noticeable farming practice that has altered in the few recent decades is the adoption of and dependence on inorganic chemical fertilizers. Sustainability of a society implies a state of economic progress in a way so as not to do any irreparable damage to the environment (Kumarappa, 1957). The following sections discuss the sustainability implications of India’s irrigation water demands in the context of fertilizer use.
Sustainability – Agriculture:
Traditional farming practices in South Asia were mainly a rain-fed operation, with two harvests in the year; the Kharfi or the autumn harvest and the Rabi or the spring harvest. The transitions between these two cropping sessions were marked with the sowing of various legumes, greens and clover to supplement the lands productivity by enriching the soil with organic nutrients. These organic nutrients are in addition to chaff and cow dung. The first monsoon rains prompted the sowing of the kharif crops and were followed with heavy rains that irrigated the fields and also brought in surface nutrients. The farmers were well aware of variability in climate and the nature (time and intensity) of the first rains revealed the season ahead. In response the farmers adapted to sowing drought-resistant (or flood-resistant) seeds and vegetables (de Boef, 2008) and were extremely knowledgeable in the use of diverse crop and seed varieties in response to varying climatic conditions. Seasonal variations, droughts and floods were not uncommon, but the traditional systems evolved to handle this. A diverse gene (seed) pool was maintained locally through sacred groves or wild-patches and the farmers were extremely adept at responding to climate variability with seeds diversity (Tripathi, 2000; Down to Earth, 2009). Sustainability was possible when the system was fundamentally able to deal with uncertainty (climate variability and change) with (genetic) diversity and adaptability (practices). Ensuring and retaining environmental (farming) vitality (crop diversity and soil nutrients) was crucial for crop yield in lean times. This was ensured through inter-cropping and organic nutrients (UN, 2003; Ladha et al., 2005). A good crop yield is particularly critical to ensure the livelihood of the majority of India’s population occupied with agriculture and allied activities. Of the 6% photosynthetic efficiency (Miyamoto, 1997), if it is assumed that the yield of grains take up 3%, the remainder of 3% of harnessed solar energy goes to the soil as nutrients, given the mixed cropping pattern followed; so practically with every harvest there is a 3% increase in soil nutrients.
Proving the “Malthusian theory” wrong has been mankind’s greatest endeavor in the green revolution – irrigation and use of chemical fertilizers. While it is difficult to debate their success in supporting a burgeoning population, they have been a classic example of weak sustainability. Weak sustainability operates on the premise that economic forces are supreme and can yield nature to predictable (immediate) outcomes (Mani et al., 2005). But, it is also known that they threaten the long-term vitality (strong sustainability) of the system, and as is evident the extensive irrigation and use of chemical fertilizers have only resulted in a drastic decline in crop diversity (regular irrigated conditions), but has also decimated soil fertility (Fukuoka, 1978; Holt-Gimenez, 2006; Harvey, 2010) and denied farmers any livelihood security. Sustainability is primarily to do with a community’s self-reliance and ability/preparedness to respond to uncertainty and change; and not associated with standardized attempts to control uncertainty and change.
Traditional Agriculture Cycle – Overview:
While India is gifted with ample rainfall in most regions, however, the rainfall is characterized by frequent heavy spells. This has particular relevance as run-offs, which are generally sediment rich, can be high and with a constant risk of flooding. The dry spells are severe and most surface-water sources run dry leaving ground water as the only source to rely on. To alleviate the conditions of frequent high rains and alternating dry spells, tradition evolved to adopt a system of interconnected lakes and ponds (Reddy and Char, 2006; NWP 2007; Frank van Steenbergen and Tuinhof 2009). These lakes and ponds acted as buffers to store excessive rains water, sumps for fertile silt and nutrients and also provided for year round supply of water during the dry season, in addition to augmenting ground water recharge. The ponds were also in close proximity, convenient and with no problems of mosquitoes as the pond ecosystem ensured a rich aquatic biodiversity that keep the larvae breeding under check. The agricultural practices were also intertwined with this system. The cycle of food production that was being followed since time immemorial is illustrated in Figure 1. When the lake/pond water levels reduced, exposing lake beds in summer, the farmers harvested this nutrient rich silt from the lake bottom to be used as organic fertilizer to enrich the soil. This helped them save on other fertilizers like leaves, compost and animal dung which was often used as a source of fuel (cowdung cakes) for cooking. The desilting of lakes, which was done every summer, year after year, possibly for tens of centuries, also helped in maintaining the storage capacity of the lakes and tanks and also provided easily accessible local nutrients to fertilize the fields. The farmers were self-reliant with their livelihood sustained well. This cycle was a closed one and thus sustainable.
Fig. 1 – Traditional farming cycle
Modern Agriculture Cycle – Overview
The demand for food production increased with a burgeoning population. This demand acutely felt in the 1960’s in India could not be immediately accommodated by the traditional farming cycle. Irrigation and modern technology devised mechanisms such as tapping deep aquifers and chemical (inorganic) fertilizers that ensured a good predictable yield; the apparent vulnerability of depending on the (uncertain) monsoon also declined along with specialized few (high yielding) of seed varieties. Native seeds meant for dry spells were rendered useless. The momentary higher yield of the chemical fertilizer in comparison to traditional farming practices (Harvey, 2010) was considered a boon and was adopted rampantly at a global scale to support a gleaming green revolution. These chemical fertilizers were subsidized by the government to encourage use and as an (industrial) economic impetus. In many regions chemical fertilizers completely replaced the use of organic and natural fertilizers. The practice of desilting of lakes and tanks beds for nutrient rich soil stopped, and in a few decades farmers were entirely dependent on the fertilizer supply regulated by the government. Unchecked drawing of ground water for irrigation was also encouraged by free electricity to operate irrigation pump-sets.
It is important to note that chemical fertilizers use petroleum products/natural gas as the primary energy and resource input material (urea) in addition to mining (phosphates) (Matson, 1997; Harvey, 2010). The photosynthetic efficiency still remains the same, except that the nutrients being supplied are inorganic and fossil fuel energy dependent. In comparison to the yield from organic nutrients the energy input in yields from chemical fertilizer use is higher by an order exceeding 65. In addition with constant application of inorganic (chemical) fertilizers made the soil sterile with no natural (nitrogen fixation) ability to generate nutrients; it only served as a sterile substrate for growing seeds on chemical fertilizers. Practically, the harvest from chemical fertilizer use is a petroleum derivative and not an organic/vegetative derivative. A comparison of nutrient values of yields from both organic and chemical fertilizer-use has clearly indicated a disparity, with the organic-fertilizer yield being far superior (Fukuoka, 1978; Edwards et al., 2007). Many experimental studies have shown that in the long-run, yields from organically fertilized lands are superior to that from chemically fertilized lands with almost 25% lower energy-input footprint (Harvey, 2010). In addition, with the adoption of the same “high-yield” seeds for irrigated fields, the pests were evolving to be specialized in infesting the crops requiring the extensive, rampant and dangerous use of pesticides.
Extensive application of these pesticides resulted in their residues flowing into lakes and ponds and disrupting the aquatic ecosystems and making them unproductive. This resulted in extensive breeding of mosquito larvae with threats to local habitations. With growing demand for land, silted and unproductive lake/pond and increasing mosquito menace, reclamation of the lakes/ponds was the inevitable consequence. With enriched local silt (fertilizer) no longer accessible and naturally unproductive farmlands, the farmers were constantly dependent on government subsidized fertilizers. In addition, with a lost local diverse seed-pool, they were further dependent on the apparently “higher yielding bio-engineered” seeds supplied by the government and/or commercial mechanisms making them completely vulnerable to externalities with their ability to be self-reliant completely decimated (Newman, 2006; Jentzsch, 2007). Thus the farming practice shifted from a closed-cycle to an ever vulnerable, unpredictable and unstable opencycle (see Figure 2.)
Figure 02 : Current Modern Farming Cycle
Agriculture and water security/availability: Current State
The persistent application and use of chemical fertilizers lead to the lakes not being deslited with unchecked flooding (and run-off of fresh water to the sea) and further impeding the storage capacity of the lakes and ground water recharge. In India, ‘the ground water table is depleting due to overdraft; water logging and salinization due to mostly to inadequate drainage and insufficient conjunctive use; and pollution due to agriculture, industrial and other human activities’ (Raju et al, 2008). In Gujarat and Rajasthan ground water over use has resulted in geogenic fluoride contamination; in Punjab, Haryana, Western Rajasthan and coastal India, ground water is increasingly saline and unfit for consumption; in West Bengal geogenic arsenic contamination is evident; and in most states in Southern India the well yields are declining rapidly, at nearly 2-3 m/year in many cities. Water levels have dropped more than 4 meters between 1981 – 2000 at the rate of 0.2m/year in nearly (Chadha , 2006).
In Karnataka groundwater utilization for irrigation has increased from 1.35 lakh hectares in 1960-61 to 8.61 hectares in 1997-98 accounting for 85-90% of groundwater use. Ground water caters to 85% of rural drinking water needs and nearly half of urban and industrial requirements in Karnataka. 36% of rural areas receive less than 55 liters per capita per day (LPCD) which is the minimum prescribed by the State Water Policy – 2002 (GoK 2004). An estimated 3 lakh wells dug in the 1970’s have run dry, and bore wells have replaced shallow open wells. Currently the state comprises two lakh drinking water bore wells and 12 lakh irrigation bore wells (as against two lakh irrigation wells around 1970’s). Nearly 50% of 234 watersheds studied in Karnataka are overexploited. A detailed study conducted in Bangalore’s Ward 39 indicates that the maximum bore well depth has increased from 200 feet in 1985 to 400 feet in 1995 and currently in excess of 500 feet. Deeper wells have resulted in geogenic contamination (fluoride, arsenic, etc.) and excessive hardness. It is interesting to note that Karnataka hails as the the seventh largest consumer of fertilizer and pesticides in India. An estimated 10-15% of pesticide application reaches the target pests, the rest being dissipated in the air, water and soil. In addition nearly 30% of the state’s tanks have lost their water holding capacity and the rate of silt deposition is an estimated 8.51 hectare meter/100 sq.km/year. This is now directly affecting water availability for hydel power generation (Raju et al, 2008).
While these incidences might look disconnected, they are in fact intricately linked within a closed-loop of human interaction with the natural system. Traditionally the farmers relied on the lakes/ponds/tanks and the open well water for domestic use. With these water sources running dry, the farmers were unable to cultivate in the dry seasons and in addition had to expend most of their productive time in fetching water from distant sources for domestic consumption. Inadequate income from cultivation, saw a shift in occupation, with the larger population of small farmers moving to nearby town and cities as migratory casual laborers with the agricultural system now running the risk of complete disruption (Chadha, 2006). Medium and large farmers could irrigate their farmlands with copious deep-well water (by installation of pumps), initially drawn from depths of 30-60 m in the 1990’s. The government stepped in to help the farmers by providing subsidies to dig bore wells and free power to operate the pumps. This provided an impetus for an unabated increase in bore-well installations and ground water withdrawal resulting in a steep decline in the water table. Pumping water from increasing depths further increases the investment for higher capacity pumps (2 kW – 10 kW) in addition to further increasing (fossil fuel dependent) electricity demand. In comparison to shallow wells, deep aquifers are usually recharged much slower (Winter et al., 1998), over centuries of water percolation through geological formations. Unrestrained extensive withdrawal saw the aquifers running dry even at depths of 200-300 m. This drying of deep aquifers is often followed by infiltration of saline water along coastal reaches which is unusable. Further, digging bore wells is a capital intensive process and many farmers simply could not afford, it even with government subsidy. The system shifted away from an open well, low rate of investment, as open wells require local labour and relatively insignificant rate of investment, that often depended on indigenous non electricity-dependent mechanisms to draw water for irrigation. Bore wells on the other hand, essentially required electricity and a much higher rate of investment. This investment was unaffordable to majority of subsistent farmers leaving them to increasing dependent on rainfall as the only source of water because the lakes/ponds/tanks have silted and the open wells dry. This threatened their livelihood and often made them victims of local money lenders charging unheard-of interest rates. To many farmers, suicide comes as a relief from further impoverishment, debt and unproductive lands (Newman, 2006; Holt-Gimenez et al., 2006). A study of the ground water depletion in three India states of Rajastan, Punjab and Haryana provides an insight into the alarming drop in ground water tables (Rodell et al., 2009). While this data applies to three states, the situation amongst other parts of India, and also possibly the world, is not very different (Hollander, 2009). The water table in villages around Bangalore (Karnataka state, India) has fallen from around 7 m to 300+ m in the last three decades (Singh et al., 2009) with most of the lands being over exploited (Raju et al., 2008, GoK, 2009) and increasingly infertile. Such consequences of a shift from traditional farming practices to extensive dependence on inorganic (chemical) fertilizer-use, though difficult to perceive, is valid, and most pronounced in rural areas (accounting for nearly 65% of India’s population).
India: food and water security
It has generally been acknowledged that the green revolution (for food security) has been made possible through the extensive adoption of chemical fertilizers. However, this has not been without consequences. Tracing the trends in agriculture over the past five decades reveal insightful observations. Figure 2 illustrates trends in grain output, irrigated area and fertilizer use in India since 1950 (RBI, 2008-09), prior to the start of the green revolution. The 1950’s saw the government’s initiatives in building irrigation infrastructure including large dams and canals. One can see that the growth in grain output started almost immediately, even prior to the adoption of chemical fertilizers.
Figure 2 (a): Trends in grain production, irrigated area and fertilizer use in India (1950-2008)
In fact the growth in grain output is more closely linked to the growth of irrigated land rather than the growth of chemical fertilizer usage. As illustrated in Figure 2(b), trends in the 1950’s and the 60’s (Fig. 2b) indicate that fertilizer use was yet to take off. During this period the irrigated area grew consistently and the food production followed this graph, clearly indicating that food grain production was more closely linked to the area of irrigated land rather than fertilizer usage.
Figure 2(b) – A closer view of the growth in fertilizer usage, irrigated land and grain output during the initial years of “green revolution”.
However, from the perspective of sustainability, the efficient performance of dam-based irrigation systems is questionable (Hussain, 2005; Rodell et al., 2009; Tilt et al., 2009; Gilbert, 2009) and would bring up another set of issues and problems that need to be addressed. Further, the argument that the per hectare yield in India is far lower when compared to countries like China, Japan, USA, etc., where the usage of chemical fertilizers are more extensive is frivolous, as the very production of chemical fertilizers is itself not sustainable given the fact that it is fossil-fuel based, as discussed earlier in this paper. Gilbert (2009) has recently highlighted the unsustainability in the use of phosphate fertilizers stating ‘Phosphate-based fertilizers have helped spur agricultural gains in the past century, but the world may soon run out of them’.
Conclusion: the vicious cycle
The paper reiterates the fact that traditional agricultural practices have moved from a sustainable “closed cycle” system, to an unsustainable “open cycle” system. The sourcing of water from deeper bore wells has lead to another set of problems as these are usually termed as fossil waters and are rich in geogenic fluorides, arsenic and other dissolved salts (Raju et al, 2008). In fact this is a grave problem faced by the country (Rao et al., 2008). For treatment and domestic use of such water, one needs to subsequently depend on reverse osmosis or chemical treatment of water which result in salt-rich residues/sludge that requires careful handling and treatment. This, needless to say, is difficult to operationalize, and the causal disposal of residual salts/sludge would progressively result in other environmental problems, viz., contamination of fresh surface-water reservoirs, toxic grounds, etc. Here, the dissolved salts that have over a few millennia remained deep in the Earth’s crust are being pumped to the surface at an alarming rate. In addition these technologies are power intensive and can be currently traced to fossil-fuel use. The process by which this system has become unsustainable is explained by the simple “arrow diagram” shown in Figure 3.
Figure 3 – Causal progression indicating consequence of increased dependence on chemical fertilizers
One can state that ‘the deeper one goes for resources, the less stable/sustainable the system’. A closed-cycle system would never leave open-ended toxic wastes/residues, and reviving the closed-cycle (see Fig. 1) system would be a solution for long-term sustainability (strong sustainability) (Orecchini, 2007). Residues from a closed-system are generally completely bio-degradable and replenishable. An important rider is that the rate of consumption should be conducive to the rate of replenishment. If not scientific logic, pure necessity is already pushing mankind in this direction. A solution to address the decreasing ground water table is to rejuvenate the lakes and water bodies as mandated by the Jala Samvardhane Yojana Sangha (Raju et al., 2008). But this requires extreme caution, as pollutants in the form of fertilizers, pesticides, lead (from electronic printed circuit boards and batteries), mercury (from fluorescent lamps), etc., would be found in these water bodies. There is an impending risk of these pollutants leeching into the ground water.
In recent years numerous debates are questioning the ability of modern industries and market economies of contributing to a sustainable world (Edwards, 2010; Latouche, 2010; Nigam, 2011). As Mahatma Gandhi observed nearly four decades back that, “Industrialization is, I am afraid, going to be a curse for mankind”. The Gandhian economist Kumarappa (1957) had propounded the Economy of Permanence defining sustainable society as one that manages its economic growth in such a way as to do no irreparable damage to its environment. Causing no irreparable damage is in effect to operate a closed-cycle.
Chadha, G.K., Indian Agriculture in the New Millennium: Human Response to Technology Challenges. In Mujumdar, N.A., and Kapila, U., (Eds) Indian Agriculture in the New Millennium: Changing Perceptions and Development Policy, Academic Foundation, New Delhi, 296-332 (2006).
de Boef, W.S., Conservation and use of farmers’ varieties. In Thijssen, M.H., Bishaw, Z., Beshir, A., and de Boef, W.S., (Eds.) Farmers, seeds and varieties: supporting informal seed supply in Ethiopia, Wageningen, Wageningen International, 125-176 (2008).
Down to Earth, Farmers' wisdom must be on paper in era of patents. Interview article Down to Earth, April, (2009). Retrieved March 2011 www.downtoearth.org.in/node/3207
Edwards, M., Small Change: Why Business Won’t Save the World, Berrett-Koehler Publishers, San Francisco, (2010).
Edwards, S., Asmelash, A., Araya, H., and Egziabher, T.B.G., Impact of compost use on crop yields in Tigray, Ethiopia, Natural Resources Management and Environmental Department, FAO of UN, Rome, (2007).
Frank van Steenbergen, and Tuinhof, A., Managing the water buffer for development and climate change adaptation, Earthscan, London, (2009).
Fukuoka, M., One straw revolution, Other India Press, Goa, (1978).
Gilbert, N., The disappearing nutrient, Nature, 461, 716-718 (2009).
GoK, Hydrology Project Phase-I, Department of Mines & Geology, Government of Karnataka, (2009). Retrieved March 2011 http://mines.kar.nic.in/hp-project.pdf
GoK, Water Resources. In State of the Enviornment Report Karnataka 2003, Department of Forests, Ecology and Environment, Government of Karnataka, 35-56 (2004).
Harvey, L.D.D., Energy and the New Reality 1: Energy Efficiency and the Demand for Energy Services, Earthscan, London, (2010).
Hollander H.M., Mull R., and Panda S.N., A concept for managed aquifer recharge using ASR-wells for sustainable use of groundwater resources in an alluvial coastal aquifer in Eastern India, Physics and chemistry of the Earth, 34, 270-278 (2009).
Holt-Gimenez, E., Altieri, M.A., and Rosset, P., Food First Policy Brief No. 12. Ten reasons why the Rockefeller and the Bill and Melinda Gates Foundations’ alliance for another green revolution will not solve the problems of poverty and hunger in sub-Saharan Africa, Institute for Food and Development Policy, (2006). Retrieved Sept 2011 www.foodfirst.org/files/pdf/policybriefs/pb12.pdf
Hussain, I., Pro-poor intervention strategies in irrigated agriculture in Asia, Poverty in irrigated agriculture: Issues, lessons, options and guidelines, ABD financed research project report, (2005). Retrieved July 2011 www.uncapsa.org/Flash/flash0705.pdf
Jentzsch, C., Patent for a pig: the big business of genetics, Documentary hosted, at www.enhancetv.com.au/ (2007).
Kumarappa, J.C., Economy of Permanence, Bhargava Bhushan Press, Varanasi, (1957).
Ladha, J.K., Pathak, H., Krupnik T. J., Six, J., and Chris van Kessel, Efficiency of Fertilizer Nitrogen in Cereal Production: Retrospects and Prospects, Advances in Agronomy, 87, 85-156 (2009).
Latouche, S., Farewell to Growth, Polity Press, Cambridge, UK, (2010).
Matson, P.A., Patron, W.J., Power, A.G., and Swift, M.J., Agricultural Intensification and Ecosystem Properties Science, Science, 277, 504-509 (1997).
Miyamoto K., (Ed.) Renewable biological systems for alternative sustainable energy production (FAO Agricultural Services Bulletin – 128), Food and Agriculture Organization, UN, (1997).
Newman, B., Indian Farmer Suicides: A Lesson for Africa’s Farmers, Food First, Institute for Food and Development Policy, 12, (2006).
Nigam, A., Desire Named Development, Penguin Books, New Delhi, (2011).
NWP, Smart water harvesting solutions, Netherlands Water Partnership, The Netherlands, (2007).
Orecchini, F., A “measurable” definition of sustainable development based on closed cycles of resources and its application to energy systems, Sustainability Science, 2, 245-252 (2007).
Raju, K.V., Manasi, S., and Latha, N., Emerging ground water crisis in urban areas – a case study of Ward no. 39, Bangalore City (Working paper 196), Institute for Social and Economic Change, (2008).
Rao, S., Mani, M., and Ravindranath, N.H. (Eds.) Advances in Water Quality and Management, Research Publishing, Singapore, (2008).
RBI, Handbook of Statistics of the Indian Economy, Reserve bank of India, Alco Corporation (for the Reserve Bank of India), Mumbai, 50-85 (2008-09).
Reddy, M.S., and Char, N.V.V., Management of lakes in India, Lakes & Reservoirs: Research & Management, 11, 227–237 (2006).
Rodell, M., Velicogna, I., and Famiglietti, J.S., Satellite-based estimates of groundwater depletion in India, Nature, 460, 999-1002 (2009).
Singh, A., Singai, C.B., Srivastava, S. and Sivam, S., Inclusive water governance: a Global necessity. Lessons from India, Transit Stud review, 16, 598-608 (2009).
Tilt, B., Braun, Y., and Daming, H., Social impacts of large dam projects: A comparison of international case studies and implications for best practice, Journal of Environmental Management, 90, S249–S257 (2009).
Tripathi, R., Implications of TRIPs on livelihoods of farmers in developing countries. Regulaciónjurídical de lasbiotechnologías, (2000). Retrieved March 2011 www.biotech.bioetica.org/docta46.htm#_ftn1
UN, Organic Fruit and Vegetable from the Tropics: Market, Certification and Production Information for Producers and International Trading Companies, (UNCTAD/DITC/COM/2003/2) United Nations, New York & Geneva, (2003).
WBCSD, Facts and Trends: Water, World Business Council for Sustainable Development, Conches-Geneva, (2006).
Winter, T.C., Harvey, J.W., Franke, O.L. and Alley, W.M., Ground Water and Surface Water A Single Resource, U.S. Geological Survey Circular, 1139, 2-10 (1998).