DEFORMACIÓN REAL Y UNITARIA

variation of rice-wheat WP is found to be different both from rice and wheat WP maps. The WP of the border areas of Rajasthan, Madhya Pradesh, and Uttar Pradesh stand out together with Indian Punjab to be the overall best performing areas examined using rice-wheat WP indicator. The shared areas of rice and wheat cultivation are influenced more by wheat than rice due to higher WP of wheat. However, areas with low wheat WP but high rice WP and the other way around are found in many areas. The rice WP contributed 50.7% to the rice-wheat WP due to larger cultivation area despite lower WP values.

Figure 4.7. Water productivity of rice-wheat rotation system in IGB and the contribution of rice to the rice-wheat system water productivity.

4.3.1. Water productivity of agricultural sector

Agricultural intensity and heterogeneity in Indo-Gangetic river basin are very high largely due to the dense population. Though, rice-wheat rotation is the major cropping pattern, other crops are also cultivated in the basin including sugarcane, oilseed crops, pulses and coarse cereals. The cultivation areas of these crops are relatively small than rice and wheat, but some have high market values hence contribute significantly to the basin WP gains.

The average WP of all kharif crops in Indian part of the basin is 0.191 US$/m3_{, which is 158% of} that of rice. Figure 4.8 illustrates the distribution of crop WP. As can be observed, the WP of all crops showed very different variation in comparison with rice WP. The “bright spots” are mainly observed in the dry areas, for example, Rajasthan, Himachal Pradesh and Uttrakhand states, where the access to water is poorer and non-rice crops are more important.

Figure 4.8. Water productivity of all kharif crops in Indian part of the Indo-Gangetic basin.

4.4. Factors Affecting Variations in Crop Water Productivity

Crop water productivity values are relatively low with tremendous variation in Indo-Gangetic river basin. These variations could be explained by many factors, for example, soil fertility, water supply, and crop management and climatic conditions. This section aims to explore the factors affecting water productivity in the basin. Figure 4.9 illustrates some important factors affecting water

productivity. The rainfall measured from Tropical Rainfall Measurement Mission (TRMM), the land surface temperature, and the ratio of ETa (actual evapo-transpiration) to ETp (potential evapo- transpiration) is illustrated separately for rice and wheat. The basin Digital Elevation Model (DEM), groundwater depth of Indian part and main river streams of Indus and Ganges are also compared. It is observed that overall rainfall during rice growing season (kharif) is much higher than that of wheat season (rabi). The rainfall in Indus basin is much lower than Ganges basin during rice growing season but higher during wheat season. Land surface temperature variations are similar in both seasons. The highest temperature is observed in the downstream areas of Indus basin where deserts dominant the landscape. And the lowest temperature occurs in the northern mountain areas. DEM shows in main agricultural areas of IGB, the land topography is relatively flat. However, the altitude of southern part of Ganges basin is relatively high. The groundwater depth to the land surface showed an obvious decline from east to the west in the basin. Large areas in Bihar and West Bengal have very shallow groundwater depth. But it becomes very deep in Punjab and Rajasthan.

Figure 4.9. Comparison of factors affecting water productivity of rice and wheat in the Indus- Gangetic basin.

Rice performance is somehow in contrast to the distribution of constraining factors illustrated above. It is observed that the rice ETp is lower in the Ganges basin, which is opposite to rainfall distribution. Significant crop stress (ETa/ETp, the lower the ratio the greater the stress) is observed in large parts of the basin, which mainly occur in Indus basin and southern part of Ganges basin. Indus is a closed basin facing severe water scarcity. Further, parts of Pakistan Sind and Punjab provinces have also witnessed significant water stress. The NDVI profiles showed that these areas have much more complicated cropping patterns in comparison to Indian Punjab. The diverse cropping types and growing periods could both lead to low average pixel ET. The Madhya Pradesh and Rajasthan states in southern part of the Ganges basin, where most severe water stress is observed, is located in high elevation and warm areas and far from the main river streams. Rainfall is also low and a large area is rain-fed. The well-performing Indian Punjab showed very little water stress in spite of deep groundwater table and low rainfall. Surprisingly, large areas in the downstream of Ganges also showed a very low water-stress level in contrast to low yields and low WP values. In these areas the rainfall is very high, which is complemented by higher flow rates in the rivers. Shallow groundwater table also directly contributes to higher evapo-transpiration.

However, higher water availability does not necessarily lead to higher yield or WP, as shown in this case. Crop development is linked to land, crop and water management practices. Rainfall may occur at any time; hence the paddy has more standing water to evaporate but could still suffer from water stress during the critical crop growth period (especially the terminal grain filling stage) which drastically affects the amount of the final grain yield. Excess water itself could also impose stress on rice growth at certain stages. Well-developed irrigation and drainage systems together with matching management practices can help maximize utilization of rainfall and river flows to achieve high yield and WP. Other land and crop interventions, such as laser land levelling, furrow- irrigated raised bed (FIRBS) cultivation, insects and diseases control, fertilizer and suitable variety, are also important factors to be considered along with water management.

Wheat ET, yield and water productivity indices are more consistent across the basin. High ET is accompanied with high yield and high WP. Rabi season is relatively dry thus the rainfall, stream and soil water availability is less significant. Irrigation becomes the main contributing factor of ET. Wheat yields are more related to irrigation volume. This explains the reason for high wheat yields in the Haryana, Indian Punjab and Pakistan Punjab: extensive irrigation is practiced in these areas, which is evidenced by a huge groundwater over exploitation zone in this area.

Both rice and wheat ETa to ETp ratio is higher in high rainfall areas. High rainfall means more water for evapo-transpiration. However, it does not necessarily lead to higher yield and water productivity, as shown from the yield maps and WP maps. This could be attributed to poor local crop and water management practices; especially the low fertilizer use, traditional varieties and incidence of crop diseases and pests. Rainfall may occur at anytime. Hence higher rainfall area has more water to evaporate but could still suffer from water stress during crop critical growth period (especially the terminal grain filling stage) which drastically affects the final grain yield accumulation. This is especially true for rice because rice yield and rice water stress (as indicated by ETa/ETp) showed contradictory trends. Wheat yield and WP follows more closely the trend of ET. As explained before, wheat relies heavily on irrigation due to the low rainfall. Hence high yield and WP is always accompanied with intensive input including water.

Further analysis revealed weather conditions as reflected by reference ET have no direct link with actual ET. The high elevation zones in the southern part of Ganges basin which is far from main river streams create problem for water access. However, large areas in flat plains and near to the main river streams also show very poor performance. The inconsistency between yield and WP distribution against the factors shows that while water availability is a major constraining factor in Indus basin downstream areas and southern part of Ganges, the main constraint in most of Ganges agricultural areas is not water availability or climate, but the irrigation infrastructure, access to

irrigation water and crop management practices. Well developed irrigation and drainage system

together with matching management practices can help to maximize utilization of rainfall and drain the excess water induced by intensive rainfall and shallow groundwater. Other land and crop interventions, e.g., land levelling, insects and diseases control, fertilizer, improved variety, are also important factors to be considered along with water management.

4.4.1 Scope for improvement of crop water productivity

Scope for improvement could be assessed in two steps: firstly through the comparison of “bright spots” and “hot spots”, secondly through site specific plant maximum photosynthesis capacity assessment. The second approach involves crop modelling for conjunctive water-fertilizer application and crop genetic innovations for high yield varieties. This is always the long term goals for the food security of the world. However, while agriculture in most developing countries is still at low level of management, the first approach provides greater chance for improved agriculture performance in near future.

The “bright spot” of both rice and wheat in Indian Punjab state and adjacent areas, with 5% of basin rice and wheat cropping area, has high WP of 0.190 US $/m3. If the basin average value of 0.131 US $/m3 _{could be increased to the same as in bright spots, the basin could theoretically} save 31% of agricultural water consumption with same quantity of production or increase 31% of production with same quantum of water input. Although this is limited by many constraining factors, a reasonable increase in WP still has a lot of significance for regional food security.

The potential for rice and wheat is different both in terms of magnitude and areas of focus. Figure 4.10 shows the plots of water productivity to yield and yield to ETa of rice. It could be observed that water productivity generally increases with increment of yield, with a relatively lower pace because ETa too increases with yield. The “bright spot” of Indian Punjab is circled. The yield of this area is so high that it totally changed the slope of yield to ETa (from S3 to S2). Some other areas

also have similar ETa, however, the yield is much lower. The scope for improvement of rice in the region will be to firstly target at the S2 trend. That is, to improve the yield with similar water consumptions. In this process the water productivity could be expected to increase 15-25%. Final target would be to increase the yield levels of all areas to “bright spot” values, during the process of which even “bright spot” might improve, which could lead to another 10% improvement.

Figure 4.10. Relations between (a) water productivity and yield and (b) yield and evapo- transpiration of rice.

The above estimated potential for improvement is under optimal conditions, which assumes no irrigation water supply constraints, soil fertility could be improved to the same level as in Indian Punjab. And land and crop management practices could all be improved at the level of Punjab. However, some constraints, for example soil, are not easy to be ameliorated. Improving yield is a long term approach to improve WP and ensure sustainable development of the region. However, in the short term, reducing non-beneficial ET from low yield areas is a practical and convenient way of increasing WP and coping with water scarcity.

The levels of WP for rice and wheat in the basin are plotted in Figure 4.11. Large coefficient of variation (CV) values, 0.44 kg/m3 _{for rice and 0.70 kg/m}3 _{for wheat, are found for both the crops.} High CV means high variability in crop WP values, indicating significant differences in performance of crop water use. CV is reduced once basin performance is more uniform. In the Indo-Gangetic basin the CV of wheat is much higher than the CV of rice, meaning water management of wheat is more diverse compared with rice. This is because the rainfall during wheat growing season is very small, hence the irrigation contribution is more significant. Increasing the irrigation management levels for wheat is probably more urgent and easier to improve basin WP.

Figure 4.11. The histogram distribution of WP values for rice and wheat.