Alternative methods to predict actual evapotranspiration illustrate the importance of accounting for phenology – Part 2: The event driven phenology model

Abstract

Abstract. Evapotranspiration (ET) flux constitutes a major component of both the water and energy balances at the land surface. Among the many factors that control evapotranspiration, phenology poses a major source of uncertainty in attempts to predict ET. Contemporary approaches to ET modeling and monitoring frequently summarize the complexity of the seasonal development of vegetation cover into static phenological trajectories (or climatologies) that lack sensitivity to changing environmental conditions. The Event Driven Phenology Model (EDPM) offers an alternative, interactive approach to representing phenology. This study presents the results of an experiment designed to illustrate the differences in ET arising from various techniques used to mimic phenology in models of land surface processes. The experiment compares and contrasts two realizations of static phenologies derived from long-term satellite observations of the Normalized Difference Vegetation Index (NDVI) against canopy trajectories produced by the interactive EDPM trained on flux tower observations. The assessment was carried out through validation of predicted ET against records collected by flux tower instruments. The VegET model (Senay, 2008) was used as a framework to estimate daily actual evapotranspiration and supplied with seasonal canopy trajectories produced by the EDPM and traditional techniques. The interactive approach presented the following advantages over phenology modeled with static climatologies: (a) lower prediction bias in crops; (b) smaller root mean square error in daily ET -0.5 mm per day on average; (c) stable level of errors throughout the season similar among different land cover types and locations; and (d) better estimation of season duration and total seasonal ET.

Evaluation of a coupled event‐driven phenology and evapotranspiration model for croplands in the United States northern Great Plains

“…Along with r 2 and RMSE, we examined the spatiotemporal patterns of residuals ( m − c ) to identify conditions where coupled scheme maintains or loses its adequacy. Previous testing of the EDPM and VegET at flux tower locations showed high levels of performance for the coupled model [ Kovalskyy and Henebry , , ]. In this regional application of the coupled model, the performance expectations are given by the earlier point‐based performances; namely, for NDVI estimates, r 2 = 0.8 ± 0.1 with RMSE = 0.1 ± 0.025, and for ET a , r 2 = 0.7 ± 0.15 with RMSE = 1.4 ± 0.5 mm per day, and transforming into 8 day aggregates by simple multiplication yields RMSE = 11.2 ± 4 mm per 8 days.…”

“…Prior to this evaluation, the EDPM was successfully tested only in point‐based experiments [ Kovalskyy and Henebry , ,]. However, the spatially explicit application of the EDPM required amendments to the functioning of the EDPM phenological phase control module in order to match the variability of the growing season dates within a wider range of conditions.…”

“…The daily K cp data were derived from the EDPM NDVI as follows: where NDVI is the EDPM‐derived NDVI defined as (). The linear coefficients in () were derived by a statistical analysis of 8 years of flux tower ET a and ET 0 measured at four flux towers (Figure ) located in Mead, Nebraska (three towers) and Bondville, Illinois (one tower) and described in Kovalskyy and Henebry []. The four towers represent typical conditions for rain‐fed commodity crops in the Central USA with maize‐soybean rotation annually.…”

“…This paper presents temporally and spatially explicit validation of the coupled EDPM‐VegET scheme that models seasonal dynamics of NDVI and evapotranspiration. Previously, the EDPM has been shown, at a small number of U.S. flux towers, to capture fine temporal details of crop NDVI dynamics and provide canopy phenology parameterization for VegET [ Kovalskyy and Henebry , ]. Similarly, the EDPM is used in this study to parameterize VegET in a regional application to estimate daily actual evapotranspiration during the growing seasons of 2007, 2008, and 2009.…”

Evaluation of uncalibrated energy balance model (BAITSSS) for estimating evapotranspiration in a semiarid, advective climate

“…Accurate ET calculation depends on the success of quantifying soil surface evaporation (E ss ) and transpiration (T) from vegetation (Ding et al, 2013;Kovalskyy & Henebry, 2012). However, even with adequate ET accuracy, most models are not accurate or unspecified in how they partition E ss and T (Decker, Or, Pitman, & Ukkola, 2017).…”