**Irrigation Supply & Demand**

(update: March, 2004)

**Seasonal irrigation supply and demand** is determined for 18
watersheds in Afghanistan. The 18 watersheds were derived from the
[GTOPO30](http://edc.usgs.gov/products/elevation/gtopo30.html) 1-km
Digital Elevation Model (DEM) using a watershed delineation tool.

The basic demand calculation is based on irrigated winter wheat water
requirements. Watershed-based volumetric water needs (**demand**) for
the period between first dekad of November and first dekad of July are
estimated based on winter wheat irrigated area estimates derived from
early 1990s Landsat data. While the spatial distribution of the
irrigated area was determined from the Landsat data, an area-magnitude
correction was applied based on the 2001/02 irrigated area estimate
conducted by FAO. The crop water use pattern is based on FAO's crop
coefficients for winter wheat with comparable agroclimatic condition in
the region.

In a given watershed, the total (seasonal) volumetric water **supply**
is determined by accumulating temporally and spatially the total
rainfall estimate (snowfall and rain) for the period between first dekad
of October and first dekad of July. Satellite-derived [rainfall estimate
(RFE)](http://edcw2ks21.cr.usgs.gov/adds/readme.php?symbol=rf) is used
from the first dekad of October to the most recent dekad (i.e., the
current dekad appearing on the graphics page). On the other hand,
long-term average rainfall is used for the time period between the
current dekad and the first dekad of July (end of season). Various types
of losses, mainly attributable to evapotranspiration and irrigation
efficiency, have been estimated as multiplicative coefficients with the
end result of reducing the total supply.

The supply/demand ratio in percent gives a measure of the degree of
correspondence between water supply and crop water demand:

Irrigation Supply & Demand = (volumetric supply / volumetric demand) \*
100

For time periods between the current dekad and the end of season, both
supply and demand are estimated using climatological (1961-1990)
rainfall and potential evapotranspiration from
[IWMI](http://www.iwmi.cgiar.org/) (International Water Management
Institute).

**Irrigation Supply & Demand Anomaly Maps:**

There are two Irrigation Supply & Demand Anomaly maps: "Median Year" and
"Previous Year".

Median Year: "Irrigation Supply & Demand Anomaly (Median Year)" shows
the relative magnitude of the current year "Seasonal Irrigation Supply &
Demand" as a percentage of the median "Seasonal Irrigation Supply &
Demand":

Irrigation Supply & Demand Anomaly (%) = (Irrigation Supply & Demand /

> Median Irrigation Supply & Demand) \* 100

where "Irrigation Supply & Demand" is current-year "Seasonal Irrigation
Supply & Demand", and "Median Irrigation Supply & Demand" is "Seasonal
Irrigation Supply & Demand" generated using the median rainfall and
evapotranspiration data from IWMI (1961-1990).

Previous Year: "Irrigation Supply & Demand Anomaly (Previous Year)" uses
the previous year's "Seasonal Irrigation Supply & Demand" instead of the
"Median Irrigation Supply & Demand".

Although site/region specific validation is important, the user may use
these anomaly maps as a semi-quantitative indicator for the
correspondence between supply and demand for irrigation purposes.