Droughts and Irrigation: Study in a River-Based Irrigation Scheme in New Zealand

Abstract

The occurrence of hydrological and agricultural droughts in a river-based irrigation scheme in New Zealand was investigated in this study. Hydrological drought occurred when irrigation supplies fell short of demands. Agricultural drought was defined by a soil moisture deficit arising from imbalances in rainfall and evapotranspiration during the irrigation season (September–April). Hydrological drought was further characterized by the number of days supplies fully or partially fell short of demands (duration) and the percentage of irrigation abstractions available when demands were partially met (severity). Based on 37 years (1972–2008) of observed river flow data (supply), it was found that one-fourth of hydrological droughts occurred early (September–December) during irrigation season. Hydrological drought started to intensify, both in duration and severity, from January and peaked in March. On the basis of climate data from 1972 to 2008, the monthly soil moisture deficit (difference between rainfall and evapotranspiration), indicators of agricultural droughts, of various return periods were analyzed. The 5- and 10-year return period agricultural droughts always exceeded the profile total available water (PAW) (estimated as a difference between wilting point and field capacity over a root zone depth of 900 mm) in the two-thirds of the irrigation scheme. Agricultural droughts peaked in January. Spatial variability in agricultural drought across the scheme, when combined with PAW, highlighted the inequity of having a single-design irrigation application rate for the entire scheme, in which large, parallel rainfall and evapotranspiration gradients exist. These gradients indicate that irrigation reliability varies spatially and temporally within the scheme.