Water and energy fluxes in an urban landscape

Abstract

Landscape planting has been designed with minimal considerations given to the plant water requirements. Little research has been done to measure the water requirements by plants in urban areas. Field measurements and the computer model ENWATBAL were used to examine water and energy fluxes of an urban landscape. Water and energy fluxes of Nu-Mex Sahra bermudagrass (Cynodon dactylon L. Pers), Raleigh St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze], wax leaf ligustrum (Ligustrum japonicum Thunb.), dwarf yaupon holly (Hex vomitoria nana L.) and Chinese elm ( Ulmus parvifolia L.), were studied for 16 days. Evapotranspiration rates for Nu-Mex Sahara bermudagrass and Raleigh St. Augustinegrass were measured with lysimeters. Daily transpiration rates for wax leaf ligustrum and dwarf yaupon holly were measured with lysimeters while hourly transpiration rates for Chinese elm were measured with a stem flow gauge. Model accuracy was evaluated by comparing evapotranspiration and transpiration rates, canopy and soil temperatures, canopy water potential and net radiation with field measurements. Daily averages of evapotranspiration for Nu-Mex Sahara bermudagrass and Raleigh St. Augustinegrass were 3.9 and 6.0 mm day'1, respectively. Transpiration rates of wax leaf ligustrum, dawrf yaupon holly and Chinese elm per unit leaf area averaged 2.5, 1.0 and 36.8 kg m '2 d a y 1 , respectively. ENWATBAL predicted evapotranspiration rates for Nu-Mex Sahara bermudagrass and Raleigh St. Augustinegrass to within 6 to 8% of measured values and ENWATBAL predicted evapotranspiration rates for Nu-Mex Sahara bermudagrass and Baleigh St. Augustinegrass to within 6 to 8% of measured values and transpiration for wax leaf ligustrum, dwarf yaupon holly and Chinese elm within 10% of measured values. The accuracy of ENWATBAL was further confirmed by its ability to predict soil surface temperature and canopy temperature within 3°C, and net radiation within 13% of the measured values. Simulations indicated that for Nu-Mex Sahara bermudagrass, latent heat flux from the soil was the dominant form of water loss, while for Raleigh St. Augustinegrass, latent heat flux from the canopy accounted for almost all of LE. Daily energy balance for wax leaf ligustrum, dwarf yaupon holly and Chinese elm showed that on average, latent heat flux from the canopy accounted for 60% of net irradiance. Sensitivity analysis showed that transpiration rates were affected most by changes in vapor pressure. Transpiration increased 1% for each 1°C increase in air tem perature.