Assessment of the Energy Balance Closure under Advective Conditions and Its Impact Using Remote Sensing Data

Abstract

A unique and intensive flux observation matrix was established during May to September of 2012 in an oasis–desert area located in the middle reaches of the Heihe River basin, China. The flux observation matrix included 22 eddy covariance systems belonging to the first thematic experiment of the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) project. The energy balance closure ratio (EBR) was assessed and possible mechanisms were investigated using remote sensing data. The results showed that 1) the EBR was in the range of 0.78–1.04 at all sites with an average EBR of 0.92, and 2) the calculated daily EBR exhibited better performance than the 30-min averages. 3) The heat storage cannot be ignored during the crop growing season. An improvement of approximately 6% in the total closure was found after considering the heat storage terms (canopy and photosynthesis storage) in the energy budget at the maize surface, and the canopy and photosynthesis showed approximately equal contributions of 3% for each storage term. The results also showed that 4) the land heterogeneous surface had a significant effect on the EBR. The EBR decreased with land surface heterogeneity increasing (taking the standard deviation of the surface temperature in the eddy covariance system source area as an index). The EBR also decreased when irrigation occurred and increased after irrigation was completed. The advection or secondary circulation broke the closed system of the energy balance given the phenomenon of EBR increasing when the advection or secondary circulation occurred.

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

“…During the period with full canopy cover (between DOY 180 and 240), the simulated T rate was consistently greater than the ET o rate (short grass reference, Figure c) possibly because of the continuous advective heat transfer and large LAI (> 4 m 2 m −2 ). Regional and local advection can cause very large evaporative fluxes in a semi‐arid climate (Alfieri et al, ; Cammalleri, Anderson, Gao, Hain, & Kustas, ; Prueger et al, ; Tolk et al, ; Xu, Ma, Liu, Shi, & Wang, ). The maximum T rate was 1.2 mm h −1 during the entire simulation period with a mean of 0.65 mm h −1 during the period with complete canopy cover (hours filtered when R s↓ < 100 W m −2 ).…”

“…During the period with full canopy cover (between DOY 180 and 240), the simulated T rate was consistently greater than the ET o rate (short grass reference, Figure 7c) possibly because of the continuous advective heat transfer and large LAI (> 4 m 2 m −2 ). Regional and local advection can cause very large evaporative fluxes in a semi-arid climate Cammalleri, Anderson, Gao, Hain, & Kustas, 2014;Prueger et al, 2012;Tolk et al, 2006;Xu, Ma, Liu, Shi, & Wang, 2017 Figure 8d). The simulated LE gradually decreased between DOY 143 and 148 (~500 to 50 W m −2 ) and subsequently increased following P events (Figure 8a).…”

Estimating Subpixel Surface Heat Fluxes through Applying Temperature-Sharpening Methods to MODIS Data

“…Consequently, monitoring and managing the water resources within the oasis during the crop growing season are urgently needed [26,27]. The HRB has long served as a test bed for integrated watershed studies, as well as land surface and hydrological experiments [28]. One major objective of the Heihe Watershed Allied Telemetry Experimental Research (HiWATER), namely the Multi-Scale Observation Experiment on Evapotranspiration (HiWATER-MUSOEXE), is to capture the strong land surface heterogeneities and associated uncertainties within a watershed [25,27,29].…”

Area-averaged evapotranspiration over a heterogeneous land surface: aggregation of multi-point EC flux measurements with a high-resolution land-cover map and footprint analysis

“…Except the lowest EBR (0.78) in orchard site, values in other sites were scattered without clear relation to the surface status. For site 15 with two heights of EC system, the relevant EBR were 0.89 (at 4.5 m) and 1.03 (for 34 m), respectively (Xu et al, 2017). For daytime conditions (global radiation Rg > 20 W m −2 ), the systematic error of the scalar fluxes (δ sys F ) can be quantified indirectly through the surface energy balance closure (EBR), which is defined as δ sys F = F · (1/EBR − 1) (Mauder et al, 2013).…”

“…1), and their representative sensible heat flux derived from a single site 4 would be insufficient. Besides, as the closure ratio of the surface energy balance (EBR) is comparatively lower over heterogeneous areas, such as for site 4 (Xu et al, 2017), the energy flux from large eddies or secondary circulations induced from surface heterogeneities may not be captured by a singlepoint EC, but may be measured via the LAS system. Thus, the LAS observations might be able to close the surface energy balance better than the EC method (Foken, 2008;Foken et al, 2010).…”