The occurrence of agricultural drought at Ashburton, New Zealand

Summary Agricultural drought is defined as existing when the soil moisture in the root zone is at, or below, the permanent wilting percentage. The condition continues until rain falls in excess of the daily evapotranspiration. The occurrence of agricultural drought at Ashburton was calculated for 44 seasons, using Thornthwaite's method of estimating changes in soil moisture. Agricultural drought occurred in every season studied, ranging from 107 days (1931‐32) to 6 days (1944‐45). The average number of drought days was 59. Nearly 60% of all days of drought occurred in spells of 10 consecutive days or more. The data obtained gave no indication of any periodicity in the occurrence of drought. The seasonal production of pasture and lucerne was negatively correlated with the total number of days of drought in a season, the correlation coefficients being ࢤ0.91 for lucerne, and ࢤ0.92, ࢤ0.58 and ࢤ0.95 for pasture measured from three experiments.