Water use efficiency in relation to crop production
Land and water scarcity are major constraints to the production of food required to meet the quantitative and qualitative shifts of the world’s demand in the mid-twenty-first century. Whereas land and water availability are constrained on a global scale, there are important regional and crop-specific differences that need to be understood, quantified, and managed. In this context, the aim of this report is to provide an assessment of water productivity using five case studies that cover major grain crops, and a broad technological range from subsistence to high-tech production systems.
Options for Improving the Productivity of Water
There are several approaches for improving the crop productivity (yields) of water including replacing high water consuming crops with lower‐consuming ones and adopting management and systems improvements to increase productivity per unit of water consumed. Reallocation of water from low‐value crops to higher‐value crops can increase the economic productivity of water; however this conserves water only if the high‐value crop has a shorter growing season, and the land is not recropped the same year. Thus, the most significant sources of “new” water will be through improvements in productivity per unit of water with the adoption of appropriate management and water application systems. Each basin and watershed may have different solutions depending on specific socioeconomic, soils, water supply and climatic characteristics.
Efficiencies must be considered in terms of both the diverted water that is consumed and the proportion that is not consumptively used that is available for reuse or becomes degraded or otherwise unusable. Efficiencies are increased when the total amount of water consumed by crops, evaporation and other users can be reduced. The available water resource within a basin or subbasin can also be effectively conserved for other uses by improving efficiencies to reduce the unusable water losses.
There may be no water savings at the watershed or regional scale if the volume of evaporation and transpiration consumed by the crops and nonbeneficial uses (e.g., weeds) stays the same (assuming no unrecoverable losses). We know that improving only on‐farm irrigation efficiencies will usually not save water, and may, in fact, increase total water use because both higher irrigation uniformities and increased inputs (e.g., fertilizer) can increase total yields. Growers may also shift to higher value, higher water‐using crops because the improved irrigation system renders it more feasible. However, improved irrigation systems do make the farmers more efficient in their overall operations and more competitive, and they are certainly a huge advantage in drought or other deficit irrigation conditions when limited water can be applied more uniformly and create better yields. Nevertheless, under drought conditions, application efficiencies may reach 100% and return flows will be low, potentially affecting riparian ecosystems.
In an urban setting, appropriate selection of drought tolerant grasses and other plants can save substantial amount of water when combined with reduced fertilizer use and other strategies to maximize use of rainfall and optimization of where and when water is applied to spatially diverse landscapes such as highway medians. This could also reduce mowing and maintenance costs while still providing an acceptable appearance. Xeroscapes and minimal irrigation of golf course roughs will likewise have a large impact on reducing urban water use.
Cultural practices such as conservation tillage, planting densities, and improved varieties and pest control will affect crop productivity and water use. Cropping strategies such as double‐cropping, intercropping, relay cropping, crop specific rotations and sequences can take advantage of the lower water demand of certain crops and times or periods with higher rainfall to increase productivity.
In addition to general water conservation and reuse practices, three complementary and overlapping options can be exercised to save water at the farm and field level. These options comprise
- Increasing crop productivity per unit of water,
- Improving management capacity of growers, and
- Spatially optimizing water applications and use.
The first option considers alternatives that include planting drought‐tolerant cultivars or reducing inputs such as fertilizers or water to decrease vegetative vigor. Option 2 can be realized by increasing grower ability to optimize irrigation amounts in time and space utilizing site‐specific irrigation techniques, enhanced water delivery systems, decision support tools, and other advanced methodologies. Option 3 can be exercised through various scientific irrigation scheduling scenarios including deficit irrigation, geographically relocating specific crops to areas of maximum adaptation and productivity, and fully or partially retiring lands from irrigation so water can be moved to more productive areas or uses.
ways and means of reducing runoff losses of irrigation water
The water requirement of crops in that quantity of water required by the crops within a given period of time for their maturity and it includes losses due to evapotranspiration plus the unavoidable losses during the application of water and water required for special operation such as land preparation, puddling and leaching. Irrigation is an important practice for higher production both in mono and poly cropped areas. Effective irrigation is the controlled and uniform application of water to crop land in the required amount at required time with minimum cost to produce optimum yields without the waste of water and any adverse effect on the soil in the form of soil salinity and water login problems. The misuse of water leads to the problem of water logging and salt imbalance thus rendering agricultural lands unproductive or less productive of problematic. As the source of water to crop land is the rainwater and supplementary through irrigation. Hence for effective use of water it is necessary to avoid water losses therefore control measure are taken for both rainwater and irrigation.
Surface mulches are used to prevent soil from blowing and being washed away. It also breaks the surface crust, which forms after each downpour. Mulching with different materials e.g. paddy husk, grass mulch, jowar stubble and wheat straw, also plastic mulches were used. Organic mulches at 5t/ha are recommended, vertical mulching with jowar straw is also recommended to conserve run-off and utilize it effectively. Mulching has advantage of to keep down weeds, improve soil structure and increases infiltration.
Irrigation should be schedule when the soil temperature is moderate. In hotter season it is better to irrigate the field during the late afternoon or at night so that both soil and plant get sufficient time to absorb water before dry and desiccating weather prevails the next day. During the very cold season water should be applying during the morning hour, when the soil starts warning up. Frequent irrigation increases application loss and cost but it helps to increase use efficiency of fertilizer and other inputs. However, how much water should be applied in each irrigation depends on the need of the crop for water, the availability of water for irrigation, the capacity of the root-zone soil to store water, the season of raising the crop, the cultural practices and the anticipated rainfall.