Environmental data
Based on the environmental requirements of GAS, the following climatic
variables were selected: annual precipitation; precipitation of driest
month, maximum temperature of warmest month and mean temperature of
coldest quarter. These were extracted from the 10 arc-minute resolution
WorldClim dataset (http://www.worldclim.org).
Precipitation data often does not reflect the amount of water input into
irrigated areas. As such, the chosen variables relating to
precipitation, annual precipitation and precipitation of driest quarter,
were modified to reflect irrigation patterns. The irrigation correction
was based on the general principle that irrigation compensates for water
loss through evapotranspiration (Brouwer & Heibloem, 1986) and uses
methods fully described in Federman et al., (2013). In summary, the
difference between evapotranspiration and precipitation was calculated
for both annual data and for data representing the driest quarter. These
differences were then applied to annual precipitation and the
precipitation of the driest quarter in irrigated areas, to give
variables adjusted for irrigation. Irrigated areas were identified from
(Siebert et al., 2005). Evapotranspiration data was extracted from
ENVIREM (Title & Bemmels, 2018), whilst the precipitation data was
extracted from the WorldClim dataset.
Multicollinearity amongst the four variables was tested for using the
Pearson correlation coefficients. A value of more than 0.7 is
considered to indicate variables which covary too much, however, this
didn’t apply to any of the four variables.