climate change on UAE - Stockholm Environment Institute-US Center

4.4. Representing Irrigation
Demands
Irrigation demand for the three main irrigated
land use types (e.g. agriculture, forests, and
amenity) in the WEAP model is driven by <strong>climate</strong>.
Irrigation requirements for a demand site are
computed using the FAO crop requirements
method, which assumes a simplified hydrological
and agro-hydrological process. Parameters and
variables used in computing crop water demand
include precipitation, evapotranspiration, crop
coefficients, irrigation efficiencies, leaching
factors, total planted area, fraction of planted
area per crop type, a crop coefficient (Kc),
irrigation efficiency, leaching factor, and fraction
planted.
Agricultural irrigation demands represent the
largest single use of water in Abu Dhabi Emirate.
In WEAP, these demands are represented for the
eastern and western region by disaggregating
farms into three representative crop types,
including ‘Date Palms’, ‘Rhodes Grass’, and
an ‘Other Crop’ category that would include
things like vegetables and grains. Evaporative
demand or Precipitation Shortfall for each crop
is estimated in WEAP based on the estimate of
PET, the crop coefficient, irrigation efficiency,
and a leaching factor. The crop coefficient,
Kc, is used to characterize the degree to which
irrigators apply water to their crops to satisfy
crop water demand. For a given crop, a Kc of
1.0 implies that 1) the crop is fully watered,
so evapotranspiration is limited by energy
availability; and 2) the evapotranspiration
rate equals that of a “reference” crop, which is
typically alfalfa.
In the Abu Dhabi Emirate, Kc values are
typically much less than 1.0, as deficit irrigation
is the norm given the scarcity of water.
Irrigation efficiency is simply the percentage
of water that is delivered to demand site but
does not serve a beneficial use to the crops.
A 90% irrigation efficiency implies that 10%
of the delivered water is lost in transmission
and/or lost to evaporation in a way that does
not benefit the plant. Seasonal variation in Kc
values is implemented to represent the reality
that winter irrigation is more substantial than
in summer, as more farmers fallow land in the
summer due to the intense heat and resulting
inefficiencies. Irrigation efficiencies were
assumed to range from a low of 70% for ‘Rhodes
Grass’, which assumed irrigation inefficiencies
through transmission and application losses,
85% for ‘Forests’ and a high of 90% for ‘Amenity’,
as trees and gardens are typically water with
hand placed, drip irrigation.
The leaching fraction represents an increase in
crop water demand to help leach salts from the
soil column. A leaching fraction of 0.2 indicates
that an additional 20% of crop water demands is
needed to help in soil-salt management. These
coefficients used in the WEAP application for
the five plants are summarized in Table ‎4‐4.
The ‘Amenity’ type is a generic category of
land cover that represents green spaces, parks,
gardens, etc. that have been planted around
the Abu Dhabi Emirate. We have assumed
that the Kc value of the Amenity land cover is
relatively high, consistent with the overall policy
of keeping those spaces green. The ‘Forests’
plant type represents planted trees that have
been established throughout the country. For
Impacts, Vulnerability & Adaptation for
Water Resources in Abu Dhabi
Table ‎4‐4. Irrigation coefficients for the five represented crops/plants.
Crop/Plant
Kc
Irrigation
Efficiency %
Leaching
Fraction %
Forests (trees) 0.2 85 5
Amenity 0.6 90 2
Ag-Date Palms 0.4 80 20
Ag-Rhodes Grass
0.7 (winter)
0.5 (summer)
70 20
Ag-Other 0.6 (winter); 0.4 (summer) 80 20
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