A Systems Approach to the Analysis of Crop Water Productivity

A systems approach to the analysis of crop water productivity

B.A.M. Bouman

Challenge Program Water and Food International Rice Research Institute, Los Baños, Philippines ([email protected])

Abstract

Key words Water productivity, water saving, resource conservation, systems analysis

Introduction

Global agriculture in the 21st century faces two major challenges. First, total production needs to increase to feed a still growing world population (ref). Since options for expansion of agricultural land are limited, increased food production has to come from increased land productivity, i.e. produce per unit of land, also simply called ‘yield’ (ref). Secondly, the increase in food production needs to be accomplished under increasing scarcity of water resources. Worldwide, about 70% of all developed fresh water resources are diverted for the irrigation of crops, making the agricultural sector the largest ‘user’ of water. Falkenmark and Rockström (2004) estimated that the current global-average consumptive water use (i.e., water used through transpiration by growing crops) for food production is 1200 m3 capita-1 year-1, but that 1350 m3 is needed to eliminate current levels of hunger and malnutrition. In order to adequately feed an estimated 9.3 billion people in 2050 (FAOSTAT, 2002), consumptive crop water use needs to increase from its present estimated level of 7000 km3 year-1 to 12,586 km3 year-1. However, fresh water resources are increasingly getting scarce. In many parts of the world, surface or subsurface water resources are overexploited, leading to rivers that run dry and groundwater tables that go down (Postel, 1997; Gleick, 1993). Industrial and domestic use of water is increasing at the expense of agricultural, although their demands are still relatively low (e.g. currently estimated at only 160 m3 capita-1 year-1, Falkenmark and Rockström, 2004). Moreover, industry and domestic users return a lot of their water in polluted form to the environment so that it is not suitable anymore for reuse downstream. Mismanagement of water in agriculture also leads to salinization in many parts of the world, which renders water or soil resources less and less suitable for agricultural production (ref). With fresh water availability decreasing and agricultural being the largest beneficiary of developed water resources, the pressure is on to save water in agriculture. The combined challenge to produce more food and saving water has lead to the notion that, rather than an increase in land productivity, an increase in water

1 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc productivity is needed, i.e., “more crop per drop” (ref). Higher crop water productivity will not only contribute to global food security but also to poverty alleviation in rural areas through higher returns per unit inputs (ref). As water is getting increasingly scarce, its opportunity cost is getting higher. As the relatively easily accessible water resources have already been exploited, the development and use of new irrigation water resources is getting increasingly expensive. Farmers will most likely share a part of these increased expenses, e.g., through the introduction of new pricing mechanisms in public irrigation systems, or directly by having to pump from deeper groundwater tables using their own shallow tubewells. Increased crop water productivities can help farmers cut back on (expensive or scarce) irrigation water inputs while maintaining yield levels and income. The increasing awareness that crop water productivity (WP) needs to be enhanced is reflected by a large number of recent publications on the topic (see e.g., Kijne et al. (2002, 2003), for recent overviews). However, there are different methods and spatial scale levels to express WP. At the level of a plant or crop, WP can be defined as the amount of biomass with economic value (for example grain biomass of cereals) over cumulative amount of water transpired (T). Since the amount of water transpired is the only water flow in an agricultural field actually passing through the crop, this water productivity can be called the “consumptive WP (WPT)”. Moreover, this transpiration depletes the available stock of water since it is no longer available for reuse at another spatial scale unit. At the level of a field, another water flow that depletes the available water stock is evaporation. For convenience sake, evaporation and transpiration are often lumped together in the term evapotranspiration (ET), also referred to as “green water flow” (Falkenmark and Rockström, 2004) or “depleted fraction” (Seckler, Molden). The “green WP (WPET)” can de defined as the amount of economic biomass over cumulative amount of water evapo-transpired. Quite often, however, the distinction between green and consumptive WP is not made (e.g., the Bessembinder paper). Increasing the green WP can be accomplished by decreasing the evaporation, which, however, does not affect the consumptive WP. On the other hand, if an increase in green WP is realized through an increase in the amount of biomass produced, then the consumptive may also be increased as well. Since in most practical field conditions evapotranspiration is hardly measured, a pragmatic way to look at WP is through water inflows rather than through water outflows. The “input WP (WPRI)” can be defined at the field level as the amount of economic biomass over cumulative amount of water inputted by rainfall (R) and irrigation (I). An analysis of WP based on outflows can result in different conclusions than an analysis based on inflows. For example, in an analysis of the effects of water- saving technologies in irrigated rice production, Bouman and Tuong (2001) found that reducing the amount of irrigation water led to increased input WP, but that it decreased yields. There was a trade-off between water and land productivity. In another example, in an analysis of supplementary irrigation in savannah and semi-arid environments, Oweis and Hachum (2003) and Falkenmark and Rockström (2004) found that the introduction of irrigation increased both the green WP and the yield, so there was a win-win situation. Although all authors carefully defined their use of WP, it is difficult to generalize and compare results, or to assess how the investigated technologies can contribute to food security and water saving. At the field level, input WP can be increased through a reduction in non-productive outflows such as seepage and percolation. Both flows, however, can be recaptured and reused further downstream. Seckler and Molden (refs)

2 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc called these outflows non-depletion flows, and dubbed any savings in water inputs caused by reductions in non-depletion flows ‘paper’ or ‘white’ savings that are not real or “blue” savings. Rather than looking at water productivity, they introduced the concept of water accounting, in which different flows of water are tracked across different scale levels throughout an entire basin. Although the concept of real water savings and water accounting are useful at the basin scale, they tend to downplay the importance of the field scale and may lead to a false sense of complacency. Why save water at the field scale through reductions in percolation, since these losses can be reused downstream anyway? Such reasoning bypasses the fact that such paper water savings at the basin scale are very real savings for the millions of shallow tubewell irrigators who can cut water costs and increase their income by reducing percolation losses from their fields. From the above brief analysis, it is clear that just “increasing water productivity” may not solve the problems of increasing food production and saving water. Nor is it easy to compare or extrapolate empirical findings from one site to another if the underlying mechanics of increased WP are not understood or quantified. A systematic framework for the analysis of WP can help in identifying generic principles, or intervention points, that can contribute to the two goals of increasing food production and saving water. The intervention points can be the improvement of genetic resources (germplasm) or of natural resource management. In this paper, a framework is proposed that can be applied at different spatial scale levels, combining the concepts of water productivity laid out above with the water accounting principles of Seckler and Molden (refs). The spatial scale levels considered are the plant, field and region or basin. The framework proposed builds on ideas developed under the Comprehensive Assessment of Water Management in Agriculture (as presented by Bennet, 2003; Kijne, 2003 – the appendix) and supports the thematic development of Crop Water Productivity of the Challenge Program Water and Food (see also Kijne et al., 200x background paper).

Analysis framework

The accepted overarching goals are to increase food production and to save water resources. The saving of water can be out of necessity if the physical availability is declining, out of economic considerations if the cost is increasing, or out of other considerations such as a societal desire to divert water out of agriculture to satisfy other needs of society (industry, domestic, nature). The above goals can be formulated as two simple objectives: o Increase crop production (under water scarcity) o Decrease use of scarce or expensive irrigation water

To identify generic strategies to fulfill the objectives, we can look at the production equation. In conventional terms, production is based on ‘land productivity’: Production = land productivity (yield) x amount of land used. Analogously, we can define production in terms of water productivity: Production = water productivity x amount of water used. In equation form: Y = WPT x Tsum (kg), where: (eq 1) o Y = amount of produce (kg)

3 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc -1 o WPT = crop produce per unit water transpired (kg produce kg water) o Tsum = total amount of transpired water (kg)

Rearranging equation 1, water productivity can be written as: -1 WPT = Y/Tsum (kg grain kg water) (eq. 2.1)

Produce is the product of total crop biomass and the harvest index: -1 WPT = [(BIOM x HI)/Tsum] (kg grain kg water), where: (eq. 2.2) o BIOM = total biomass of the crop (kg) o HI = harvest index of the crop (-)

BIOM is the accumulation of daily net photosynthesis products: -1 WPT = [(PHOTOsum x HI)/Tsum] (kg grain kg water), where: (eq. 2.3) o PHOTOsum = sum of net photosynthesis (kg)

Transpiration (Tsum) is the amount of water withdrawn from a certain storage pool (water in the rooted soil profile), which can be characterized by its water balance:

W = Inflowsum - Tsum - other Outflowsum (kg), where (eq. 3.1) o W = change in stored water o Inflowsum = sum of all water inflow components o Other outflowsum = sum of all water outflow components besides transpiration

Re-arranging equation 4.1 we get equation 4.2:

Tsum = Inflowsum - other Outflowsum - W (kg) (eq. 3.2)

Examples of inflows are irrigation and rainfall and examples of other outflows are evaporation, deep percolation and surface runoff. Crop growth and transpiration are dynamic processes and need to be considered over time. The amount of water available for transpiration is not only determined by the amounts of daily inflows and outflows, but also by the size of the storage pool. This size determines the ‘buffering capacity’ of the system: a surplus of water inflow can be stored and be available to the crop at later times when there is a deficit of water inflow. Taking this buffering capacity into account, equation 4.2 becomes:

Tsum = Cs(Inflowsum – Outflowsum) - W (kg), where (eq. 3.3) o Cs = Storage coefficient (-)

Combining equations 1, 2 and 3 we get a generic description of crop production in terms of water use:

Production = [WPT ] x [Tsum ] =

Production = [WPT ] x [Cs (Inflowsum – Outflowsum) - W] =

Production = [Y/Tsum] x [Cs (Inflowsum – Outflowsum) - W] (eq. 5)

4 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc water productivity term (1) storage size term (2) inflow term (3) non transpiration outflow term (4) storage change term (5)

The objectives to i) maximize crop production (under water scarcity), and ii) minimize the use of scarce/expensive irrigation water can be realized by the following four generic “principles”:

1. Increasing consumptive crop water productivity (WPT) = increasing the produce (Y) per unit water transpired (Tsum) 2. Increasing the storage size (in time or space) 3. Increasing the non-irrigation water inflows 4. Decreasing the non-productive (i.e., non-transpirational) water outflows

Water for transpiration can also be met by exploiting and depleting the storage pool (W is then negative), but, on the long run, this is unsustainable. The analysis of crop production and water scarcity can be addressed at different spatial scale levels, for example plant, crop, field, irrigation system, catchment and basin levels. A systems approach is useful in exactly defining the terms of the production equation and identifying strategies to implement the generic principles at the different scales. In a systems approach, the boundary conditions define the components of inflow and outflow, the nature and size of the storage term, and determine which of the flow rates are internally or externally determined. In the following sections, production equation 5 and the four generic principles are elaborated for three systems: plant, field, and region or basin.

Plant level

For an individual plant, equation 5 can be written as:

Production = [(PHOTOSYNTHESISplant x HI plant)/Tstomata] x

[Cs (root water uptake – Tcuticle) + W], where (eq. 6) o Tstomata = transpiration through leaf stomata o Tcuticle = transpiration through leaf cuticle

The system boundaries are formed by the outside of the plant above and below ground (Figure 1). The storage unit is the plant itself. The only water inflow into the system is water taken up by the roots and the only outflow is water transpired, which can be divided into transpiration through the stomata (Tstomata) and transpiration through the cuticle (Tcuticle). The only internal water flow is vertical (upward) movement of water in the plants from the roots to the leaves. The four principles to increase production and

5 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc minimize irrigation water can be implemented only through genetic improvement of the germplasm (Table 1).

Plant 1. Increasing consumptive crop water productivity (WPT) Bennet (2003) recently presented an overview of options to increase WPT through genetic improvement. There are basically two possibilities: increasing the photosynthesis per unit transpiration through the stomata, or by increasing the harvest index. Only the water transpired via the stomata can be termed as truly productive outflow since CO2 enters the same stomata to be converted into biomass via photosynthesis. For given crop species, the ratio of photosynthesis over stomatal transpiration is usually considered to be constant with little scope for improvement by breeding (check Ismail). However, knowledge about stomatal regulation and its underlying genetic mechanisms is rapidly increasing which may lead to breakthroughs in the future. Peng et al. (1988) demonstrated that some improved tropical japonica lines of rice have higher rates of photosynthesis over transpiration than older indica varieties, suggesting that breeding may have played a role in improving this characteristic. Another improvement in WPT may be the conversion of C3 plants into C4 plants through genetic engineering. C4 plants are more efficient in their photosynthetic pathway and have a higher water use efficiency than C3 plants. Recent thoughts and advances on achieving C4 photosynthesis in the C3 plant rice have been reported by Sheehy et al. (2000). The varieties of the green revolution have demonstrated the success in increasing the harvest index through breeding (Khush, 2001). The harvest index can be increased by manipulation of the plant architecture, namely ‘dwarfing’, and by increasing the amount of assimilates produced to the plant organ of economic interest, such as the grains in cereals. The dwarfing genes for cereals have been successfully identified and may be exploited to obtain similar gains in harvest index in other crops (Bennet, 2003). A special consideration is a plant’s ability to maintain a high harvest index under biotic stress conditions such as drought, salinity or extremely high or low temperatures. Though drought affects the plant’s growth at many stages of its development, drought at flowering is generally recognized as having most detrimental effect on yield (through reduced harvest index) for many crops (Boonjung an Fukai, 1996; Zinselmeier et al., 1999; Saini and Westgate, 2000 (all in Bennet paper). Bennet (2003) gives an overview on recent advances in, and prospects for, genetic improvement of harvest index under various abiotic stresses.

Plant 2. Increasing storage size Although certain cacti and orchids do have an efficient system of internally storing water (cacti throughout their body, and orchids through their special spongy root structure), there hardly seems an opportunity to increase the internal water storage of agricultural plants through breeding.

Plant 3. Increasing the non-irrigation water inflows The plant does not distinguish between the source of water at its roots, so there are no options to increase specifically non-irrigation water uptake.

Plant 4. Decreasing the (non-productive) water outflows

6 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc Water transpired directly through the cuticle is not directly productive, although it contributes positively to crop growth through cooling and maintaining the sap flow that transport nutrients and biosynthetic products from roots to leaves. Options to decrease cuticular transpiration are to increase the resistance to water flow, for example by increasing the waxiness of leaf surfaces (although more understanding of the water loss through the cuticle needs to be gained).

Field level

For a farmer’s field, the generic equation 5 can be written as:

Production = [(BIOMcrop x HI)/Tcrop] x

[Cs ((I + R + C + Sin + Ron)-(E + Tweed + Sout + P + Roff)) + W] (eq. 7)

The system boundaries are the top of the crop and the bottom of the rooted zone in the vertical plane, and the field boundaries in the horizontal plane (Figure 2). The storage unit is the rooted soil volume plus any storage place on the surface of the soil. The water inflows into the system are irrigation (I), rainfall (R), capillary rise from groundwater table (C), lateral subsurface inflow (Sin) and run-on (Ron). The water outflows from the systems are evaporation from soil (or, in the case of paddy rice, ponded water) (E), transpiration by the crop (Tcrop), transpiration by weeds (Tweed), lateral subsurface outflow (Sout), deep percolation (P) and run-off (Roff). Internal water flows are horizontal and vertical movement of water in the root zone, and horizontal and vertical movement of water over the soil surface, and vertical (upward) movement of water in the plants (from root to leaves). The four principles to increase production and minimize irrigation water can be implemented by both improvements of the germplasm and of natural resource management practices (Table 2). Of course, the options at the individual plant level remain valid at the field level as well.

Field 1. Increasing consumptive crop water productivity (WPT) There are no specific options to improve the WPT of germplasm at the field level other than those at the plant level. Although there are many options to increase biomass production through improved management of natural resources, e.g. water and nutrients, the ratio of biomass production over transpiration has been shown to be fairly constant for a given species in a given climate under various levels of nutrient and water inputs (de Witt, 1958? See refs by Falk and Roc). However, there are options to increase WPT at the field level by shifts in planting dates. A crop experiencing dry and hot weather will transpire more than the same crop experiencing cool weather conditions (based on thermodynamic principles, a high temperature and a large vapor pressure deficit all promote transpiration). Since biomass production may be the same in cool and in hot conditions, it has been argued that planting crops in the cool season may increase the WPT over planting the same crops in the hot season (Tuong, 1999), though this concept still needs validation. Such a shift in cropping pattern holds implications for cold tolerance of the varieties used (see also “Field 3” below).

7 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc Good crop protection against pests, diseases and lodging is an often overlooked strategy to safeguard high levels of WPT. If a crop gets devastated at the end of the growing season, little yield may be harvested despite that a lot of water has been used (transpired) to produce the crop. Some calamities affect the whole crop (e.g., rat damage or lodging) whereas some specifically reduce the economic produce (e.g., stem borer in cereals). Following the same reasoning, reducing post-production losses contributes to safeguarding high levels of WPT. (to give some data to exemplify orders of magnitudes).

Field 2. Increasing storage size Increasing the root length of plants is an efficient breeding strategy to increase the storage size by allowing crops to tap water stored in deeper soil layers. In breeding, this strategy is known as “drought avoidance” (Blum, 1988). For rainfed lowland rice, the capacity of roots to penetrate the hardpan underneath the puddled layer at times of drought may be a special breeding target (Ray et al., 1996 (Bennet paper); Len Wade papers). Management options to increase the storage size are the breaking of hard pans and deep ploughing or soil ripping (refs?). Fertilizer application has been shown to increase root length and depth (e.g., Cooper and Gregory, 1987 (in Debaeke paper).

Field 3. Increasing the non-irrigation water inflows Varieties with longer roots may be able to capture shallow ground water either through direct uptake (if the roots are partly within the groundwater) or through capillary rise (Boling et al., in prep; Belder et al., 2004). The development of short duration varieties shortens the length of the growing season and helps is escaping late or end-of-season drought. In breeding, this strategy is known as “drought escape” (Blum, 1988), and has been one of the most successful mechanisms to increase crop production in drought- prone environments (Bennet, 2003?). Although not more rainfall is captured in absolute terms, in relative terms more rainfall is captured per growing day. More options exist, however, to increase the amount of non-irrigation water inflows by management techniques. More rainfall can be captured by adjusting the cropping pattern to the rainfall season. In wet-season rice, direct dry seeding can advance the growing season and utilize early-season rainfall more effectively than transplanted or direct wet-seeded systems (Cabangon et al., 2002; Tabbal et al., 2002). In the semi-dry regions of West Asia and North Africa (WANA), shifting from summer cereals to winter cereals allows the efficient use of winter and early spring rainfall (Oweis and Hachum, 2003). This shift, however, has implications for breeding since cold-tolerant varieties need to be available for this strategy. Besides adjusting the cropping pattern to the rainfall pattern, a whole array of options exists to capture rain water more efficiently, especially in the semi-dry or savanah areas (see a recent overview by Debaeke and Aboudrare, 2004; more refs). For example, the crop density can be adjusted to make optimal use of rainfall distribution. Crops can be planted in slight natural depressions where rainfall accumulates through within-field runoff. Also, within-field run-off water may be redirected to flow to areas where crops are planted (within-field water harvesting). Whole field, sections of fields or even individual plants can be bunded to capture rainfall and make it better available to the crop (this strategy may also be seen as increasing water storage).

8 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc There are two irrigation strategies do not increase the amount of non-irrigation water use in absolute terms, but are aimed at minimizing irrigation water use, and may thereby increase the relative proportion of non-irrigation water use. With supplemental irrigation, small amounts of irrigation are carefully applied at critical times of the growing season when rainfall is insufficient (Oweis and Hachum, 2003). In deficit irrigation, less irrigation is applied than required to realize maximum crop production (Oweis and Hachum, 2003; Zhang, 2003).

Field 4. Decreasing the (non-productive) water outflows Breeding crop varieties that are effective weed suppressants contributes to the reduction of non-productive transpiration by weeds (Bennet, 2003). Developing herbicide-resistant crop varieties is another appropriate breeding strategy. Management options to reduce weed infestation are the use of herbicides, mechanic and manual weed control, crop rotation, soil tillage and phytosanitation measures. Non-productive water outflows can be especially high in the tropical climates of semi-dry and savanna areas of the world. Falkenmark and Rockström (2004) estimated that in the semi-arid tropics of sub-Sahara Africa, about 10-25% of rainfall flows out of a field as runoff, 10-30% as deep percolation, 30-50% as evaporation, and only 15-30% as consumptive crop transpiration. Technologies to reduce these unproductive outflows are generally labeled “conservation farming practices” (e.g. overviews by Hobbs and Gupta, 2003; refs). Mulching reduces evaporation and runoff, and promotes infiltration of rain water into the root zone. With light rainfall, plowing and harrowing the soil reduces runoff and promotes infiltration. With heavy or high-intensity rainfall, plowing can promote surface sealing and have the opposite effect. In that case, zero or minimum tillage are better alternatives. Deep soil ripping with minimum topsoil disturbance again promotes infiltration and deep rooting by the crop. The on-farm water harvesting techniques described earlier to increase the inflow of rain water also reduce the evaporation and runoff outflows. Deep percolation is an especially large outflow from irrigated paddy fields. Sharma (1989) estimated that percolation accounted for 50-80% of the total water input to the field. Management measures to reduce percolation are alternate wetting-and-drying, direct seeding, saturated soil culture, soil compaction, land leveling, and aerobic rice (Bouman and Tuong, 2001; Tuong and Bouman, 2003). Good crop management contributes indirectly to the reduction of unproductive outflows. A strong and healthy growing crop produces al large amount of biomass that makes transpiration a strong “competitor” for water relative to the other outflows. Breman et al. (2001) and Oweis and Hachum (2003) showed that an increased nutrient applications increased biomass growth, crop water uptake and green crop water productivity (WPET). Falkenmark and Rockström (2004) reasoned that increased green crop water productivities were mainly realized by a vapor shift from less evaporation to more crop transpiration. A fast growing crop not only suppresses weeds, but also shades the soil surface faster, thereby reducing soil evaporation. In his review on resource use efficiency in agriculture, de Wit (1992) re-emphasized the validity of Liebscher’s law of the optimum, which states that a factor which is in minimum supply [water] contributes more to production the closer other factors [such as nutrients] are to their optimum.

9 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc Regional or basin level

For an entire region or water basin, the generic equation 5 can be written as:

Production = [(BIOMcrop x HI)/Tcrop] x

[Cs ((I+R+RI+Sin+Ron)-(Ef +Ew+Tweed+Tnatural+Sout+RO+P+Roff)) + W] (eq. 8)

The system boundaries of a basin are the top of the crop and other vegetation and the bottom of aquifers carrying extractable water in the vertical plane, and the hydrological divide in the horizontal plane (Figure 3). The storage in the whole system can be divided into “primary” and “intermediate” storage. The primary storage is the total of all rooted soil volume (plus any water stored on its surface) that holds the water that can be taken up by the crop for transpiration. The intermediate storage pools temporarily store water for later release to the primary storage pool. Examples of intermediate storage are lakes, rivers, and underground aquifers. The water inflows into the system are irrigation (if this comes from outside the system area) (I), rainfall (R), river inflow (RI), lateral subsurface inflow (Sin), and run-on (Ron). The water outflows from the systems are evaporation from soil or ponded water on fields (Ef), evaporation from open water bodies such as reservoirs and canals (Ew), transpiration by the crop in the field (Tcrop), transpiration by weeds in the field (Tweed), transpiration by non-agricultural vegetation such as trees, reeds, bushes etc. (Tnatural), lateral subsurface outflow (Sout), river outflow (RO), deep percolation into sinks below the system (P) and run-off (Roff). Internal water flows are horizontal and vertical movement of water in the root zone, between the root zone and the other water aquifers in the system, over the soil surface, between the various secondary storage elements (reservoirs, canals, aquifers), and upward movement of water in the plants, weeds and trees (from root to leaves). The four principles to increase production and minimize irrigation water can be implemented mainly by improved natural resource management.

Basin 1. Increasing consumptive crop water productivity (WPT) There are no specific options to increase WPT at the basin level.

Basin 2. Increasing storage size The creation of intermediate storage is probably the most important option at the basin level. Reservoirs may vary in size from small on-farm ponds or cisterns for supplementary irrigation (Falkenmark and Rockström, 2004), to village level ponds or tanks such as in Sri Lanka and Tamil Nadu in India (also called “off-farm water harvesting”), to large reservoirs created by building small or large dams that service up to hundreds thousands of irrigated area. Though these reservoirs vary in size, they all serve the purpose of storing river or rain water in times of surplus to make it available for in times of deficit. Besides these surface reservoirs, underground aquifers can also be exploited as intermediate storage. Secondary reservoirs are effective mechanisms to capture and reuse drainage water. For example in the 160,000 ha Zanghe Irrigation System (ZIS) in Huibei, China, farmers have build thousands of on-farm and village level ponds to capture surface drain water coming out of paddy field from higher elevations (ref). These reservoirs have probably greatly contributed to maintaining the volume of rice produced in ZIS despite the fact that agricultural water allocations from the ZIS

10 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc reservoir dropped from about 80% in the mid-sixties to around 25% in the early nineties (Loeve, Dong). Farmers are also smart in exploiting natural or artificial water ways and shallow groundwater aquifers as secondary storage. For example in District II of the Upper Pampanga Integrated River Irrigation System (UPRIIS) in the Philippines, about 20% of all water delivered through the main canal is recycled by farmers who pump drainage water from small creeks and percolated water from shallow groundwater (IRRI, unpublished data).

Basin 3. Increasing the non-irrigation water inflows There are no options to increase non-irrigation water inflow across the hydrological divide. Trans-boundary transfer is done by canals or pipelines and is therefore an (expensive) irrigation water flow.

Basin 4. Decreasing the (non-productive) water outflows Traditionally, irrigation systems have been developed by redirecting water flows away from natural systems such as wetlands and forests, thereby decreasing green water flows from these ecosystems that were considered non-productive. However, there is an increasing realization that these natural ecosystems perform a vital role in the life-support system of our planet (Falkenmark and Rockström, 2004). Any development of irrigation must go hand in hand with balancing the available fresh water resources between agriculture and nature. Besides these “users’, a balance must also be struck with other users such as aquatic ecosystems (e.g., river water that supports fish production), industry and cities. Even water that flows out of the basin may perform a vital role at the next spatial unit. For example river water outflows into the ocean may play a crucial role in maintaining mangroves, navigable river entries, limiting salt sea water intrusion, or protection of the land against the sea by providing the sand for dune formation. Tracking water flows and water uses and reuses from upstream to downstream through a basin is crucial to ensure that any reduction or redirection in “non-productive” water flows does not adversely affect a particular user to an unacceptable level.

Conclusion and Discussion

The analytical framework and the four key principles presented here are useful to identify key intervention points towards maximizing crop production and saving scarce or expensive water resources. They can help analyze and understand the underlying mechanics of technologies that enhance crop water productivity at different spatial scale levels. This, in its turn, can help identifying extrapolation domains for such technologies and estimate their full potential in meeting future food demands through a wise use of water. The few examples presented here to illustrate the framework are far from exhaustive. They do, however, disentangle complex underlying relationships. Sometimes, a technology incorporates more than one key principle. A case in point is within-field water harvesting that reduces the non-productive outflows run-off and evaporation and increases the inflow of rainwater into the root zone for subsequent crop transpiration. The examples also show that added value of combining technologies, such as the introduction of water harvesting with supplementary irrigation (Falkenmark and Rockström, 2004).

11 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc Another example is the combination of breeding with natural resource management strategies, such as early winter sowing of cold-resistant varieties of wheat in semi-arid areas of the WANA countries (Oweis and Hachum, 2003). The usefulness of the conceptual framework can be strengthened when quantitative measurements are made of the various water flows in the system under study. Another step is to dynamically model the water flows and production functions to assist ex-ante analysis of of proposed interventions (see e.g. Bessembinder et al. (xxxx) and Bouman et al. (2005) for examples at the field level, and Khan et al. (refs) for examples at the irrigation system level).

Acknowledgements

This paper greatly benefited from discussions with T.P. Tuong, John Bennet and Jacob Kijne.

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Figure captions

Figure 1. Water flows at the plant level. Abbreviations are explained in the text.

Figure 2. Water flows at the field level. Abbreviations are explained in the text.

Figure 1. Water flows at the regional or basin level. Abbreviations are explained in the text.

13 4/26/2018 0f796030aa676260923d5837e3ec4c7a.doc Table 1. Plant level options

Principle Specific measure

Water productivity  Photosynthesis  : keep stomata open, C3  C4 HI  : lodging resistance, improved assimilate partitioning, increased heat and cold tolerance, reduced spikelet sterility (drought), increased salt tolerance, short duration Storage  - Non-irrigation inflow  - Other outflow  Decrease Tcuticle (waxy leaves)

Table 2. Field level options

Principle Specific measure Plant Field Water productivity  - Y : good crop husbandry (nutrient, weed, water, pest management) Storage  Deep root system Deep puddling, deep plowing; break obstructing soil layers, high/good bunds Non-irrigation inflow  - Early establishment and dry-seeding to use rainfall, capture overland flow, intercept seepage inflow Other outflow  Weed suppression Decrease E: evaporation suppressant; beds, AWD, aerobic rice (zero till)

Decrease Tweed: weed control Decrease P: SSC, AWD, beds, aerobic rice, soil compaction, good puddling; crack plowing Decrease S: bund maintenance; (as in decrease P)

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