Abstract
A study was carried out to parameterize the METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) model for estimating instantaneous values of albedo (shortwave albedo) (αi), net radiation (Rni) and soil heat flux (Gi), sensible (Hi) and latent heat (LEi) over a drip-irrigated Merlot vineyard (location: 35°25′ LS; 71°32′ LW; 125 m.a.s. (l). The experiment was carried out in a plot of 4.25 ha, processing 15 Landsat images, which were acquired from 2006 to 2009. An automatic weather station was placed inside the experimental plot to measure αi, Rni and Gi. In the same tower an Eddy Covariance (EC) system was mounted to measure Hi and LEi. Specific sub-models to estimate Gi, leaf area index (LAI) and aerodynamic roughness length for momentum transfer (zom) were calibrated for the Merlot vineyard as an improvement to the original METRIC model. Results indicated that LAI, zom and Gi were estimated using the calibrated functions with errors of 4%, 2% and 17%, while those were computed using the original functions with errors of 58%, 81%, and 5%, respectively. At the time of satellite overpass, comparisons between measured and estimated values indicated that METRIC overestimated αi in 21% and Rni in 11%. Also, METRIC using the calibrated functions overestimated Hi and LEi with errors of 16% and 17%, respectively while it using the original functions overestimated Hi and LEi with errors of 13% and 15%, respectively. Finally, LEi was estimated with root mean square error (RMSE) between 43 and 60 W∙m−2 and mean absolute error (MAE) between 35 and 48 W∙m−2 for both calibrated and original functions, respectively. These results suggested that biases observed for instantaneous pixel-by-pixel values of Rni, Gi and other intermediate components of the algorithm were presumably absorbed into the computation of sensible heat flux as a result of the internal self-calibration of METRIC.
Highlights
Nowadays drip irrigated viticulture in semi-arid zones is under political and economic pressure to optimize irrigation strategies, considering the persistent diminishing water supply, due to increased consumption, mismanagement and pollution
The volumetric soil water content at rooting depth ranged from 0.25 to 0.32 m3·m−3 and mean Ψx was between −0.4 and −1.0 Midday stem water potential (MPa) indicating that the Merlot vineyard was not under water stress during the three growing seasons [32,65,66,67]
35 (±50), 43 (±53) and −15 (±31) W·m−2, respectively; while those using the original functions presented biases of 29 (±33), 33 (±40) and 2 (±35) W·m−2, respectively. These results suggest that errors in the estimation of LAI_M, zom_M and G_M were balanced by the self-calibration of METRIC to estimate the instantaneous turbulent energy fluxes
Summary
Nowadays drip irrigated viticulture in semi-arid zones is under political and economic pressure to optimize irrigation strategies, considering the persistent diminishing water supply, due to increased consumption, mismanagement and pollution In this regard, irrigation strategies such as regulated deficit irrigation or partial root drying have emerged as potential ways to increase water savings [1,2,3]. Systems, which have been widely used in research to quantify ETa of a vineyard [7,12,13,14,15,16,17]. These systems do not provide spatial trends of ETa because these measurements represent a point influenced by a small footprint area [18,19]
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