The estimation of seasonal soil moisture deficits and irrigation requirements for Ashburton, New Zealand

Abstract

Abstract A survey of supplemental irrigation requirements for Ashburton, New Zealand, was carried out for the September to April period of the years 1912–3 to 1955–6. Day-by-day changes in soil moisture deficits were calculated, using daily rainfall figures and mean daily estimated evapotranspiration values for each month. The frequency and distribution of two irrigation treatments were determined. Loss of rainfall as drainage was estimated by crediting to drainage all rain which occurred while the soil was at field capacity. It was found that where irrigation was applied to prevent the soil moisture deficit from increasing beyond 1.2 in., an average of 7.6 irrigations per session was required. The range in the number of irrigations required was from 4 to 11. The amount of drainage averaged 7.90 in., which was 38.9 per cent of the rainfall received. The average “effective” rainfall (calculated by subtracting the amount of drainage from the total rainfall) was 12.40 in., 61.1 per cent of the total rainfall. The average seasonal soil moisture deficit which was replenished by irrigation was 9.54 in. The addition of this to the “effective” rainfall gave a total water requirement for an average season of 21.94 in. If irrigation were applied to prevent the soil moisture deficit from increasing beyond permanent wilting point (a deficit of 2.4 in.), an average of 3 irrigations per season was required. The range in number of irrigations required per season was from 1 to 5.