Water Flux in a Cashew Orchard during a Wet-to-Dry Transition Period: Analysis of Sap Flow and Eddy Correlation Measurements

Abstract

Information regarding biosphere?atmosphere interactions is important in the study of a hydrological cycle. To this purpose, xylem sap flow (SF) using the Granier system and evapotranspiration (ET) using the eddy correlation method were measured during a ?wet-to-dry? transition period in a young cashew (Anacardium occidentale L.) plantation. Estimates of half-hourly tree transpiration made from stem sap flow measurements and above-canopy eddy correlation measurements of water vapor flux were compared for a period of 22 days of complete records. Cross-correlation analysis was used to estimate the time lags (τ) between the time courses of SF and ET, and between SF and ET with solar radiation (RS) and vapor pressure deficit (D). Applying a simple functional technique, values of τ = 43 min (November), τ = 46 min (December), and τ = 75 min (January) with an overall τ = 53 min (using all data) between the time courses of ET and SF were estimated. A positive lag indicates that SF lags behind ET. However, both ET and SF were more dependent on RS (r2 > 0.81) than on D, whereas SF was more related to D (r2 = 0.60) compared to ET (r2 = 0.38). An insignificant (p > 0.05) decrease in daily values of both ET and SF over the 22 days of concurrent measurements were observed. Daytime average ET ranged from 2.01 to 3.17 mm day?1 with a mean of 2.7 mm day?1, whereas values of SF ranged from 0.55 to 0.72 mm day?1 with a mean of 0.65 mm day?1. Tree transpiration accounted for about 25% of the evapotranspiration from the orchard. This result may be of help in correctly predicting the diurnal behavior of transpiration from sap flow measurements.