Thursday, August 9, 2007

PS 68-195: Estimating the soil water resources needed for revegetation, using Baccharis pilularis

Matthew J. Curtis and Victor P. Claassen. University of California, Davis

One of the problems revegetation experts face is the question of how much soil water is needed in order to sustain the desired vegetative community. Plant water use can be approximated by multiplying potential evapotranspiration by a reducing factor for the plant type, and then also by a stress factor for a variable plant response to drought, depending on seasonal moisture conditions. Traditional agro-meterological methods used to measure water use of field crops (ETc) are based on assumptions that are difficult to satisfy when in native plant communities. Despite this, the use of the traditional method (Penman Monteith-ETo) is desirable because of its wide use and its ability to use local climate data to determine site specific water use patterns. We measured the crop water use (ETc) of Baccharis pilularis (Coyote bush, a native revegetation species in California) and determined its crop coefficient (Kc, where ETc/ETo = Kc). Moisture probes were placed in the soil to determine at what percent of the plant available water the plants began to reduce their water use. This information was used to mark the beginning of the crop stress when the crop stress coefficient (Ks; the ratio of a water stressed plant compared to a well watered plant) changed from 1.0 to a value between 1 and 0. The model (ETc = ETo*Kc*Ks) was used to predict the daily water use of coyote bush based on local climatic conditions and soil water resources. The water use of non-watered coyote bush plots agreed with model predictions as plant water use decreased during the summer. From this plant behavior data, soil treatments can be designed that provide adequate moisture to grow plants through summer dry seasons without supplemental irrigation.

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