Exploring the Implications of Modeling Choices on Prediction of Water
Savings

Improvements in irrigation technology are expected to yield water
savings. Recent research highlights the need for accompanying
institutional conditions (e.g., restricting irrigation expansion).
However, estimating the expected quantity of water savings remains
uncertain, even under such institutional conditions. This is because
estimates of the water savings resulting from improved irrigation
technology are subject to several methodological (sometimes arbitrary)
choices. Three key choices are: (1) the underlying hydrologic model used
to partition irrigation water into consumed (e.g., evapotranspiration)
and non-consumed (e.g., runoff) components, (2) the selected hydrologic
model parameters, and (3) the convention used to represent
non-beneficial losses (e.g., non-crop evaporative losses during channel
conveyance, on-farm application, off-farm storage, or unrecoverable
seepage). This study is the first to explore the combined implications
of these choices as regards predicting water savings. It is also the
first to attribute the uncertainty in expected water savings to each of
these choices. To explore these implications, we use an ensemble of
water savings under all possible combinations of three different
conceptual hydrologic model structures (HYMOD, HBV, SAC-SMA), a hundred
equifinal parameter sets (for each model), and two conventions for
representing non-beneficial losses - a total of 600 scenarios. The
results show that parameter selection and alternative conventions of
representing non-beneficial losses are the largest sources of
uncertainty in water savings, contributing ~49% and
~33% respectively to overall uncertainty. These results
provide a quantitative estimate for the minimum range of uncertainty one
may expect when considering policy options that depend on quantified
estimates of water savings.