Evaluation of irrigation water amount in semi-arid croplands using time series of FORMOSAT-2 images

Irrigated agriculture is the largest consumer of water worldwide, and especially in semi-arid areas such as southern Mediterranean countries. Agricultural water use is difficult to estimate over large areas, since several irrigation networks may co-exist and since farmers operate private pumping stations. In this context, the objective of this work was to estimate irrigation through the combination of a crop/water balance model and optical remote sensing data. The methodology was tested on a 2800 ha irrigated area located in the Tensift-Marrakech plain, central Morocco. Remote sensing data consists in the time series of FORMOSAT-2 images. This sensor operates in four bands (blue to near infrared) and allows to acquire daily images at 8m spatial resolution. Images are used in conjunction with a simple crop model to monitor the nature of crops, their phenological states and the average level of water stress. This information and other ground data (soil map, climatic data) are introduced into a water balance model, to estimate irrigation at the pixel resolution. The results are evaluated over irrigation units (25 to 200 ha size) with data collected by the regional agency in charge of the distribution of dam water. At a seasonal scale, the accumulated irrigation water was found to vary between 0 and 300 mm, with accuracy of 25%. The approach may offer perspectives for a better estimate of the quantity of ground water used for agriculture. Indeed, it works without requiring any a priori data on agricultural practices. This makes it very attractive for operational application at a regional scale.