Another key modification in this model is the introduction of
river basins. An earlier version of the model employed agroecological
zones (AEZ) in recognition of the fact that significant
climate and soil variations within economic regions require more
refined spatial units for modeling. However, the presence of
irrigation water, drawn from a given river basin, further
complicates this picture. When AEZs cut across river basins,
production conditions may differ, even within the same AEZ.
Therefore, the GTAP-BIO-W model allows crops to compete for
land within the AEZ, in addition to a second layer of competition in
which irrigated cropping activities compete for irrigation water
within a river basin. The total water available for irrigation is
exogenously specified in each river basin