Loss in monthly pasture production resulting from dryland conditions in the Waikato

Abstract

This paper examines a simple model requiring monthly estimates of the maximum weather-dependent evapotranspiration and daily rainfall to predict the reduction in monthly dry land pasture growth relative to irrigated production. The basic assumption is that dry matter production is proportional to transpiration. The model is tested using results from two irrigation experiments on ryegrass-white clover pastures rotationally grazed by beef cattle. These trials took place in the central Waikato Valley on two soil types; an Aquic Hapludult (HAM) from 1953 to 1964 and a Typic Vitrandept (HOR) from 1962 to 1966. For this period dryland conditions reduced yields over the November through April period by 39 and 27% respectively on the two soils. Predicted monthly dry land yields using irrigated yields and the estimated water use showed no systematic bias compared with actual pasture production and accounted for 83% and 77% of the yield variation for the HAM and HOR soils respectively. Errors caused by the lack of fit of the model combine with those of measurement and the spatial variability of pastures so that the standard deviation of a predicted monthly yield is about 250 kg DM ha-1 • Errors attributable to the model are of a similar magnitude to those of conventional pasture yield measurements. The model is not sensitive to the details of the transpiration-soil water storage relationship assumed. Limitations and applications are discussed.

A practical model for predicting soil water deficit in New Zealand pastures

“…Further work is required to determine the dependence of a on pasture and soil types. If a can be successfully related to pasture species, this may provide a means to study the relative response of alternative pasture species to drought conditions (Parry et al 1992), as plant stress is closely related to ^£7 (McAneney et al 1982;McAneney & Judd 1983;Parfitt et al 1985aParfitt et al , 1985bRosenthal et al 1987;Barker et al 1989;Sadras & Milroy 1996). This would then allow a practical model of plant growth response to water stress to be formulated.…”

“…43 Scotter et al (1979a used the Priestley & Taylor (1972) PET model, and proposed a linear relationship between soil water deficit and AET on both theoretical and observational grounds. A similar approach has been used by other authors (Baier & Robertson 1966;McAneney et al 1982;, and has been shown by Calder et al (1983) to work well in comparison with other AET models. However, like other published AET models, the Scotter et al (1979a) AET model requires soilspecific empirical constants, which can only be obtained through detailed trial work at the site in question, or potentially from published values, which have not been forthcoming.…”

“…By including a zone which is depleted and recharged preferentially, the Scotter et al (1979a) model offers an ingenious solution to the problem encountered by single-layer water balance models (e.g., Baars et al 1977;McAneney et al 1982), namely that full-profile models under-predict pasture transpiration and growth following short-lived re-wetting of the topsoil during a prolonged dry period. Furthermore, because the two zones in the Scotter et al (1979a) model overlap, water movement between, or water extraction from, the two zones does not need to be modelled explicitly.…”

“…Whereas a number of proven models exist for (weather limited) potential evapotranspiration, PET (Priestley & Taylor 1972;Coulter 1973;Smith et al 1996), models of actual (soil water and/or vegetation limited) evapotranspiration, AET, have generally been empirical, and only locally applicable (Baier & Robertson 1966;Calder et al 1983;de Jong & Bootsma 1996). AET models for use in pasture are less problematic in this regard, as the "crop" canopy is usually closed, meaning that evaporation from the bare soil can be neglected (McAneney et al 1982), and a mature root network may be assumed (cf. Baier & Robertson 1966).…”

Pasture production and water use measurements in the central Waikato

“…To further examine Equations (1) and (2) previously assumed by McAneney et al (1982), we present measurements of dryland and irrigated pasture yields along with neutron probe measurements of the soil moisture status. In the Waikato, summer rainfall exhibits a marked between-seasonvariability.…”