Seasonal variabilities in the distribution of the microclimatic factors and evapotranspiration in a shortgrass steppe

Ecosystem models designed to simulate the long-term dynamics of soil-vegetation-atmosphere interactions require information on the properties of the land surface and the temporal pattern of the microclimate variables. This paper describes the seasonal distributions of microclimatic factors in a semi-arid shortgrass steppe and a potential method for estimating evapotranspiration in sparse vegetation. The shortgrass steppe is located in the piedmont of north-central Colorado. Microclimate measurements were taken between 1986 and 1989. Precipitation showed wide variability between the years and could significantly account for the seasonal differences in growth and biomass yield of the native vegetation. Small (0.5 cm or less) precipitation (rain and/or dew) events, determined from the positive increments in hourly lysimeter measurements, contribute 8-10% to the total annual precipitation at the site. The total net radiation (R(n)) was found to be 47% of the total incident solar radiation. The ratio of surface soil heat flux to R(n) (G(s)/R(n)) was found uniformly equal to 0.167 during the vegetated season (April-October). Midday sensible heat flux (H) was found to Ix: linearly related to the difference between the temperatures of the soil measured at 0.025 m depth and the air above the canopy. The ratio of H and R(n) taken at midday was found to be linearly proportional to the ratio of the daily totals of H and R(n). A method for estimating daily evapotranspiration (ET(day)) based on the energy balance approach was developed. This method was found suitable for estimating ET(day) in the shortgrass steppe under high soil moisture conditions and/or on days following rainfall. For potential application of this method of estimating ET(day) to dry soil conditions, integration of plant transpiration controls into the ET(day) equation is suggested.