Continuous Measurement of Soil Evaporation in a Drip-Irrigated Wine Vineyard in a Desert Area.

Evaporation from the soil surface (E) can be a significant source of water loss in arid areas. In sparsely vegetated systems, E is expected to be a function of soil, climate, irrigation regime, precipitation patterns, and plant canopy development, and will therefore change dynamically at both daily and seasonal time scales. Current understanding of E in vegetated systems is limited due to the lack of robust techniques to measure E continuously. In this study we assessed two novel techniques, a soil heat balance method and a surface temperature based method, for continuous measurement of E in a drip irrigated vineyard in an arid environment. Specific focus was assessing variations of E both temporally and spatially across the inter-row. Continuous above canopy measurements included evapotranspiration, solar radiation, air temperature and humidity, and wind speed and direction. Short-term intensive measurements below the canopy included E and solar radiation along transects between adjacent vine rows. The intensive measurements were used to validate long-term assessment of E based on soil heat pulse probes and infrared thermometer data combined with existing evaporation models. Below canopy E was highly variable; both diurnally and with distance from the vine-row, as a result of shading and distinct wetted areas typical to drip-irrigation. While the magnitude of actual E was mostly determined by soil water content, diurnal patterns depended strongly on position relative to the vine-row due to variable shading patterns. Results indicate that continuous E data can be obtained from temperature method, provided the below canopy micro-climate can be adequately modeled. The soil heat balance method can be used as a stand-alone technique and is suitable to assess second stage E. The strengths and weaknesses of each approach will be highlighted.