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UBC Theses and Dissertations

Development of a climate-based forage growth model for a Peace River community pasture Wallis, Charles Hubert


Based on periodic clipping of a fertilized pasture plot in the Peace River region in 1977 and 1979, accumulated dry matter production of a timothy, red fescue and alsike clover mix was found to be linearly related to accumulated transpiration during the active growing season, with a growth/transpiration ratio of 0.026 t ha⁻¹ /(mm H₂O)-The effect of fertilizer level and cutting management on dry matter production is discussed. Energy balance/Bowen ratio measurements of evapo-transpiration (E) in 1977, 1978 and 1979 showed that daytime E can be calculated for energy limiting conditions using the Priestley-Taylor formula with α = 1.26 ± 0.05. Daytime net radiation required in this formula was estimated to within 15%, using the Idso-Jackson longwave radiation equation and daily solar radiation data from a regional climate station 50 km away. During water supply limiting conditions E was found to be linearly related to root zone water storage. Root zone drainage was found to be negligible in this soil, which has a high bulk density subsoil. A simple model for calculating the course of pasture growth during the growing season at Sunset Prairie Community Pasture is described. The model is composed of a single-layer root zone water balance submodel and a relationship between dry matter production and transpiration. The water balance submodel estimates daily transpiration and requires daily values of rainfall, solar radiation and maximum and minimum air temperature. It also requires crop albedo and an estimate of the initial root zone water storage. Estimates of root zone water storage during the three growing seasons agreed well with gravimetric and neutron moisture probe measurements. The model, using the above growth/transpiration ratio, was found to estimate hay growth during the droughty growing season in 1978 to within 15% of measured values. An effective growth/transpiration ratio of 0.013 t ha ⁻¹/mm was required to account for the growth of pasture subjected to a simulated monthly grazing rotation.

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