Irrigation requirements and the hydrological regime of some Auckland soils

Abstract

Eighteen months of weekly and bi-weekly soil water data and daily meteorological data were used to derive four water balance simulation models. These were developed for the associations Otao silt loam-pasture, Otao silt loam-kiwifruit, Patumahoe clay loam-pasture, and Patumahoe clay loam-kiwifruit. The structure of the models is: S(i+1) = Si + Ri - AETi - Di - ROi where S(i+l) is soil water storage at day i+1, Si is soil water storage at day i, R is rainfall, AET is actual evapotranspiration, D is drainage at the base of the soil horizon and RO is runoff. In each model a specific function was developed to simulate AET, D and RO. The models were run with 31 years of daily meteorological data (1954-1984) to produce 31 years of daily soil water content estimates. The resulting four soil water data sets were used to determine the moisture regimes and irrigation requirements of the corresponding soil/plant associations. Water uptake properties (root uptake pattern and transpiration characteristics) and ground water recharge were also investigated. The form of each of these functions was entirely determined from the data available. Actual evapotranspiration (AET) was found to be dependent upon both potential evapotranspiration (PET) and soil water content (SWS). Soil water storage determines whether AET is equal to or less than PET. The higher PET is, the higher soil water storage needs to be so that AET = PET. This is expressed formally as follows:If PET > AETmax then AET = AETmax Else AET = PET AETmax can be seen as the maximum rate at which the soil can convey water to the roots. It is a positive, linear function of SWS (although it was used in a discrete form in the models), and varies from 0.1 mm/day up to the prevalent PET rate. Three semi-empirical expressions for the drainage function were inferred from theoretical knowledge of soil hydraulic properties. One of these expressions was used in the soil moisture regime models with the appropriate parameters and performed better than purely empirical functions, particulaly at high values of SWS. The prediction performance of each model was assessed quantitatively by comparison with the 18-month data sets. The prediction error was shown to be within up to about +/- 1 cm of water 95% of the time, equivalent to about +/- 1% of the water storage of the soil horizons. The 31-year daily soil water data sets produced were analysed to quantify the soil moisture regime under each plant/soil combination, and obtain information about the occurrences of soil moisture deficit and corresponding irrigation requirements. Soil water regime was quantified by determining the probability of occurrence of any soil water storage value for each day of the year. Accordingly, the probabilities of occurrence of annual and monthly irrigation volumes of all magnitudes were determined. The maximum depth from which water is taken by kiwifruit was found to be about 2 m in both soils, against about 1 m for pasture. The storage capacity of readily available water is a little larger in Patumahoe clay loam (up to 0.11 v/v) (v/v=volume by volume) than in Otao silt loam (up to 0.08 v/v). Soil moisture deficits occur every year in pasture, and about 9 years in 10 under kiwifruit. Deficits are mainly concentrated in December to March inclusive in pasture, and January to April inclusive for kiwifruit. Irrigation is required every year for pasture, and about 9 year in 10 for kiwifruit. Its monthly distribution is similar to that of soil moisture deficit occurrences. The annual irrigation requirements are 240 mm for kiwifruit on Patumahoe clay loam (Pukekohe) and 185 mm for kiwifruit on Otao silt loam (Kumeu), with a return period of 3 years in 4. At a return period level of 1 year in 2 the irrigation requirements are 145 mm and 105 mm respectively. For pasture, the annual irrigation requirements are 265 mm at Pukekohe and 170 mm at Kumeu with a return period of 3 years in 4, and 190 mm and 145 mm respectively with a return period of 1 year in 2. Distributed over the 120 days of the irrigation season, the irrigation requirements for kiwifruit amount to 1.5 mm/ day at Kumeu and 2 mm/day at Pukekohe, with a return period of 3 years in 4. These results are not very different from the current official guidelines of 2.5 mm/day. However the irrigation season is better defined, the irrigation values are much more precise, including probability distributions, and adequate storage facilities can now be designed with much more confidence.