Managing Soil Water to Improve Rainfed Agriculture in India.

摘要

Rainfed agriculture is practiced on two-thirds of the total cropland area of 162 million hectares (Mha) in India. While yield and total productivity of irrigated crops have improved since the 1960s, those of rainfed crops or dry farming have stagnated. Yet, the average crop yield under rainfed conditions in research and demonstration plots is two to four times higher than the national average crop yields. Low crop yields under rainfed conditions are due to recurring drought stress, high soil temperatures, widespread soil degradation and desertification, and poor management. Soil-related constraints that exacerbate drought stress include crusting and compaction, low water infiltration rate, low water retention capacity, high surface runoff, and high losses due to soil evaporation. India receives about 400 x 10(6) ha-m of rainfall annually, most of which is received in 100 hours over a span of 25 nonconsecutive rain days. Thus, 45% (or 180 x 10(6) ha-m) is lost as runoff or blue water. Some of the water stored in soil as green water is lost by soil evaporation, and the productive green water used as direct transpiration is rather small. Impedance to deep root penetration due to high bulk density, low porosity, and hard-setting are among important factors responsible for low fraction of productive green water. Recommended management practices (RMPs) that conserve water in the root zone and increase water use efficiency (WUE) are: (i) plowing of deep compacted soils with massive structure and low porosity, (ii) using a minimum or no-till system in light-textured soils with favorable structure, (iii) mulching with crop residues, other biosolids, and synthetic polymers, (iv) harvesting runoff water and recycling it for supplemental irrigation, and (v) adopting integrated farming systems involving legume-based rotations and agroforestry measures, which reduce water runoff and improve soil fertility. Available research results suggest a large potential for improving productivity through adoption of site-specific RMPs. Thus, there is a strong need to validate RMPs on benchmark soils in diverse agro-ecoregions of India, under on-farm conditions and with farmer participation, to facilitate widespread adoption.