Net irrigation water requirements for major irrigated crops with variation in evaporative demand and precipitation in southern Alberta

Abstract

Weather variability has a profound influence on crop and irrigation water requirements. Estimates of crop evapotranspiration (ETc) and net irrigation water requirements are needed for water allocation, risk management and irrigation system planning. Seasonal ETc and net irrigation water requirement estimates based on the standardized Penman-Monteith method were examined through frequency analysis of historical weather data. Seasonal ETc calculated using the Penman-Monteith equation is based on daily solar radiation, air temperature, relative humidity, and wind speed. Historical weather data from 1983 to 2012 at Lethbridge and Vauxhall were used to determine seasonal ETc, seasonal precipitation and net irrigation water requirements for 11 major (most prevalent) irrigated crops in southern Alberta. Seasonal ETc was consistently greater at Lethbridge than Vauxhall, whereas seasonal precipitation was generally less at Vauxhall than Lethbridge for all major crops. Mean seasonal ETc ranged from 355 mm for barley silage at Vauxhall to 728 mm for alfalfa hay at Lethbridge at a 10% chance of exceedance. Mean net irrigation water requirements ranged from 273 mm for barley silage at Vauxhall to 526 mm for alfalfa hay at Lethbridge at a 10% chance of exceedance. Area-weighted seasonal ETc demand within the irrigation districts is currently about 500 mm (2.8 billion m3) and the net irrigation water requirement within the irrigation districts is at least 380 mm (2.1 billion m3) at a 10% chance of exceedance. Annual gross diversion requirements for the irrigation districts could approach the licensed water allocation limit of 3.45 billion m3 at a 10% chance of exceedance when conveyance losses, irrigation system application efficiencies, and current irrigation management practices are considered. The frequency with which annual gross irrigation water requirements approach or exceed this licensed water allocation limit may increase in the future with climate change in southern Alberta.