The downstream impacts of increasing water consumption in the upstream rain-fed areas of the Karkheh
Basin, Iran are simulated using the semi-distributed SWAT model. Three scenarios are tested at subbasin
and basin levels: converting rain-fed areas to irrigation agriculture (S1), improving soil water availability
through rainwater harvesting (S2), and a combination of both (S3). The results of these scenarios were
compared against the baseline period 1988–2000. The S1 scenario shows a 10% reduction in mean annual
flow at the basin level, varying from 8–15% across the subbasins. The reductions in mean monthly flows
are in the range of 1–56% at the basin level, with June witnessing the highest flow reduction. Flow reductions
are comparatively higher in the upstream parts of the basin, as a result of a relatively higher potential
of developing rain-fed areas coupled with comparatively lower amount of available runoff. The impacts
of S2 are generally small with reductions of 2–5% and 1–9% in mean annual and mean monthly flows,
respectively. The results of S3 are in general similar to those of S1. Although the estimated annual flow
reductions remain well within the available water resources development potential, measures needs to
be taken to avoid excessive flow reductions in May, June and July. It is recommended that only a limited
agricultural area should be converted from rain-fed to irrigated agriculture (about 0.1 million ha),
and should practice supplementary irrigation. The supplies should also be augmented through developing
additional water storage. Adopting such measures is extremely important for the upper subbasins
Gamasiab and Qarasou where comparatively higher flow reductions were estimated.