CSEI Explainer: Crop Water Requirement
Understanding how much water a crop needs can clarify what demand-side water management strategy is more suitable.
Illustrations by Aparna Nambiar
In many parts of semi-arid India, there simply isn’t enough water for all farmers to grow remunerative crops like paddy or wheat all year round. Many of the programmes being implemented to ensure rural water security focus on the supply side, i.e. building water conservation structures or rainwater harvesting. But this is not sustainable.
There has been a recent push to invest in demand-side interventions (agricultural practices and crop choices that use water more prudently), with several government programmes now requiring water security plans prior to introducing interventions. However, knowing precisely where we need demand-side instead of supply-side and what kind of demand-side intervention is most suitable for a region depends on many factors. This is hampered by lack of access to the right data and technical expertise.
This is one expertise bottleneck that we hope to eliminate with tools such as Jaltol.
Read | Our Vision for Jaltol
Jaltol, a free, open-source water accounting tool we launched last year, uses remote sensing data — rainfall, evapotranspiration, soil moisture, surface water/groundwater storage, land use/land cover — together on one platform in an easy-to-use format.
In this blog post, we explain why understanding how much water a crop requires for its growth is critical to choice of demand-side management strategies and how tools like Jaltol can help bridge this understanding.
Identifying the right strategy
Demand side interventions range from: a) more drastic, market-based changes — increasing the market value for low water-consuming crops and driving crop choices towards the same, to b) more conservative, technological changes — promoting drip and sprinkler based systems, or perhaps c) process changes — mulching or irrigation scheduling.
How can we determine which strategy or combination of strategies is best suited to a certain geography? One way is to examine the potential water savings that can be achieved via each strategy. This means understanding:
a) Crop Water Consumption (CWC) — this is the status quo, i.e. how much water crops in a given region are actually consuming, versus
b) Crop Water Requirement (CWR) — how much water the same crops in the same region actually need.
and comparing these estimates with the amount of water available in the same region.
It might be the case that the existing cropping pattern is not particularly water intensive and that larger savings can be achieved by simply adopting technological or process changes. Alternatively, in some places, it could be the case that the potential water savings achievable via technological changes are simply insufficient and cropping pattern changes are the need of the hour.
These are questions that can be answered specifically for different geographies via a tool like Jaltol.
Crop Water Requirement & Irrigation Requirement
The methodology we use in Jaltol to compute CWR is a well established and well known methodology developed by the Food and Agricultural Organisation (FAO) of the United Nations. Through agricultural studies done over several decades, it is possible to estimate the ideal CWR for a number of crops grown across different ecological regions in different seasons. The main formula used, from a bird’s-eye view is simply the following:
Where ETc is the Crop Water Requirement, ETo is the water required by an average crop under particular climatic conditions and Kc is the multiple of the specific crop being considered. i.e. for instance, if Kc is 2, the specific crop consumes twice as much water as the average crop and if Kc is 0.5 it consumes half as much water.
Once we know the Crop Water Requirement, we can estimate the Irrigation Requirement by simply adding the quantity of water required to saturate the soil and replenish percolation losses.
Within Jaltol, a civil society organisation (CSO) can simply choose a village or watershed, input a known cropping pattern, and then compute the ideal Crop Water Requirement and Irrigation Requirement for the same area in million cubic metres (m3) of water. This serves as a benchmark number.
Crop Water Consumption
The Crop Water Consumption in Jaltol is developed using a remote sensing dataset called SSEBop (Simplified Surface Energy Balance). The SSEBOP model is an energy balance model that uses a number of weather parameters (solar radiation, air pressure, temperature, relative humidity, wind) to estimate Potential Evapotranspiration i.e. the maximum possible ET in a region given the current climatic conditions. The Land Surface Temperature (LST) is then used to estimate the actual evapotranspiration as a fraction of the potential evapotranspiration.
In Jaltol, we take the SSEBop estimate of Evapotranspiration and compute the Crop Water Consumption by multiplying it with the fraction of land that is agricultural land. This gives us an estimate (in m3) of water consumed by agriculture in the village or watershed.
Read | What is ‘Agri-Rain’? Field Notes From Anantapur
If Crop Water Consumption is much much greater than Crop Water Requirement in a region it is an easy indicator that there is large potential for water savings via demand-side interventions.
As water is spread across multiple sectors (drinking, industrial, and commercial) with competing interests, using evidence-based approaches for better water demand management, as described above, will be necessary. We wish to partner with CSOs and government institutions, to develop case studies demonstrating how useful this approach is.
If you represent a CSO or a government agency that is involved in watershed work, please reach out to us at jaltol@atree.org.
