US: Research aimed at combining up-to-date weather and satellite data within a water balance model to estimate irrigation that is more precise is demonstrating promising result. Lars Pierce, a researcher at Cal State University, Monterey Bay, and his colleagues at the National Aeronautics and Space Administration, Ames Research Center, have been working on using Landsat satellite imaging with irrigation. The project is funded through the California Department of Water Resources and NASA.
Using the satellite imagery, Pierce gets the normalised difference vegetation index (NDVI) by comparing red reflectance to near-infrared reflectance in the plants. “When you take a ratio of the red to infrared, then that directly correlates with crop densities or plant densities,” Pierce said. “What we do with that information then, we can get at the density of the crop. Water use is directly related to crop density.”
He noted that the researchers can deduce directly from the satellite imagery an estimate of the crop coefficient. The crop coefficient is a dimensionless number (usually between 0.1 and 1.2) that is multiplied by the evapotranspiration value (ETo) to arrive at a crop evapotranspiration estimate.
The California Department of Water Resources has a network of weather stations throughout California called the California Irrigation Management Information System.
“CIMIS provides that ETo at each station on a daily basis, so if you know your crop coefficient, and you know your ETo, then you can multiply the two and get an estimate of crop water use for that day, or for that week, or for the month, whatever you’re interested in,” Pierce said. He added, “The satellite imagery used to cost USD 500 a scene, but now it is free, and it is available at the USGS (United States Geological Service) Glovis website (glovis.usgs.gov).”
According to Pierce, a website is being set up that will be similar to Google Maps. A grower can zoom in, click on a particular block and find the crop coefficient. “You could scroll through many different days over the course of the season and see, just by clicking on your particular block, how your crop coefficient changed,” Pierce explained.