Implications of Model Uncertainty on Predicting the Benefits of Irrigation Investments

Water savings can be expected to result from improved irrigation technology, only when accompanied by certain institutional conditions (for example, restrictions on expanding irrigated lands). However, even under such conditions, estimating the expected quantity of such savings in individual cases remains uncertain. 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 conceptualization used to partition irrigation water delivered to the farms soil into consumed (evaporation, crop transpiration) and non-consumed (runoff, infiltration) components, (2) the equifinal set of hydrologic model parameters, and (3) the convention used to represent off-farm losses (e.g. evaporative losses during channel conveyance or off-farm storage). This study is the first to explore the range of uncertainty in the prediction of expected water savings associated with each of these choices. The adopted methodology generates a small ensemble of predicted 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 off-farm losses. The 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 resulting from investments in improved irrigation technologies.