OASIS

(Options AnalysiS in Irrigation Systems) is a model that simulates irrigation interventions in medium to large-scale irrigation systems (thousands of ha), describes return flows and integrates recycling. The model performs simulations over an irrigation season or a year to evaluate the impacts of alternative land uses, infrastructure improvements and alternative water management practices on water use, crop production and overall irrigation performance.. Each simulation takes place under specific environmental conditions, including climate (rainfall and reference evapotranspiration) and water resource availability.

The model is innovative in its systemic representation of an irrigation project (Figure 1) which includes groundwater and drainage system components. The groundwater system captures seepage from canals as well as deep percolation fluxes. It interacts with the drainage system, which collects runoff fluxes and canal spills. The included components and interactions provide OASIS with several novel features, notably:

• The ability to capture all the main factors of the water balance within an irrigation project, including non-process depletion from fallow lands and natural vegetation (non-irrigated land uses can be included as part of the upper soil compartment in Figure 1);
• The ability to capture and manage recycling of irrigation return flows, thus allowing conjunctive canal, groundwater and drainage water use.

Figure 1. Synoptic view of the OASIS model components

Taking the value of return flows into account, OASIS makes it possible to evaluate the true efficiency and productivity of water, thus discarding the misconceptions associated with the “classical” concepts of efficiency. This model feature represents an essential step towards integrated management of water resources in a basin context.

The core of OASIS is a structure built on three nested levels, namely system, irrigation unit (IU) and field (Figure 2). OASIS defines a system as the command area of a main canal (or a main canal network) diverting water from a single source such as a river, a reservoir, a pumping station or another irrigation canal.

To account for the large-scale spatial variability of soils, land use, infrastructure, water availability and management, OASIS models an irrigation system as a set of interdependent geographical objects referred to as irrigation units. Each IU identifies the right or left side of the command area pertaining to a delimited reach of main irrigation canal. IUs are characterized in a lumped fashion, assuming that nothing is known about where features are located or how they are spatially distributed within their boundaries. Accordingly, IUs define the actual spatial resolution of the model. OASIS associates a distribution, groundwater and drainage system to each IU.

To account for the smaller-scale, intra-IU variability of soils, land use, irrigation practices and water availability, OASIS associates each IU with a set of fields. Fields are abstract, elementary land use units, each characterized by specific soil and land cover properties. They are not located in space and are modeled as 1-dimensional entities. Fields are generated by the model from the lumped description of IUs so as to provide an adequate representation of the soil and land use variability in these IUs. It is at the field level that the soil-crop-water interactions are simulated.


Figure 2 The nested, three-level structure of OASIS

The model was developed by Dr. Nicolas Roost (formerly of IWMI). It is coded in Delphi (Figure 3). The model with the user manual is available for free and can be obtained from Dr. Xueliang Cai (X.Cai@cgiar.org ). Consultations on model functionality may be provided by Dr. David Molden (d.molden@cgiar.org).