Remote sensing can be used as a very efficient tool for managing
irrigated agricultural areas. A number of different applications can be
used for operational management and planning for irrigated schemes and
usually it is necessary to develop local adaptations to existing
methods. Applications include crop mapping during the growing season,
assessment of water abstraction and assessment of crop yields and
productivity. It is usually an important component in water resource assessment.
Water abstraction - semi bio-physical vegetation productivity
approach
An assessment of vegetation photosynthetic
activity will be made at different points in time. Vegetation water
consumption will then be determined as a function of vegetation
photosynthetic activity expressed as a function of NDVI and/or fAPAR
(fraction of absorbed photosynthetically active radiation) and weighted
relative to the per cent vegetation cover of the irrigated fields. Three
times during the growing season (using three different satellite
scenes) the accumulated water consumption will be estimated from
different vegetation production and vegetation cover measures and
evaluated against reference data.
Water abstraction - empirical approach
From
optical remote sensing data different measures of wetness using mid- and
near- infrared information can be used, e.g. the Tasselled Cap
transformation. Radar data can equally provide different information on
humidity. Together with the vegetation growth information derived from
NDVI and/or fAPAR, a statistical analysis will identify possible
empirical models for water abstraction. Multiple regression analysis
will be used. The results will be compared with the reference data.
Crop Yield
The approach of the well known light
use efficiency model (LUE) will be tested (Kumar & Monteith 1982,
Rasmussen, 1998). This can be done using fAPAR as an input to assess
crop yield and using single high
or medium
resolution remote sensing data (maximum three different images).
Implementation
Capacity building is an important
part of GRAS's activities. The implementation activity includes the
transfer of technology and methods to the client. This will be done
through on-the-job training and more formal training about the used
methods and techniques to relevant staff at the client's organisation.
Once this is achieved the trained local remote sensing staff will make
their own first assessment of water abstraction and crop yield for a new
growing season. GRAS will provide technical backstopping and if
necessary, follow-up missions can be arranged for solving problems,
supervision and quality control.