Abstract
Abstract. Thermal and multispectral remote sensing data from low-altitude aircraft can provide high spatial resolution necessary for sub-field (≤ 10 m) and plant canopy (≤ 1 m) scale evapotranspiration (ET) monitoring. In this study, high-resolution (sub-meter-scale) thermal infrared and multispectral shortwave data from aircraft are used to map ET over vineyards in central California with the two-source energy balance (TSEB) model and with a simple model having operational immediate capabilities called DATTUTDUT (Deriving Atmosphere Turbulent Transport Useful To Dummies Using Temperature). The latter uses contextual information within the image to scale between radiometric land surface temperature (TR) values representing hydrologic limits of potential ET and a non-evaporative surface. Imagery from 5 days throughout the growing season is used for mapping ET at the sub-field scale. The performance of the two models is evaluated using tower-based measurements of sensible (H) and latent heat (LE) flux or ET. The comparison indicates that TSEB was able to derive reasonable ET estimates under varying conditions, likely due to the physically based treatment of the energy and the surface temperature partitioning between the soil/cover crop inter-row and vine canopy elements. On the other hand, DATTUTDUT performance was somewhat degraded presumably because the simple scaling scheme does not consider differences in the two sources (vine and inter-row) of heat and temperature contributions or the effect of surface roughness on the efficiency of heat exchange. Maps of the evaporative fraction (EF = LE/(H + LE)) from the two models had similar spatial patterns but different magnitudes in some areas within the fields on certain days. Large EF discrepancies between the models were found on 2 of the 5 days (DOY 162 and 219) when there were significant differences with the tower-based ET measurements, particularly using the DATTUTDUT model. These differences in EF between the models translate to significant variations in daily water use estimates for these 2 days for the vineyards. Model sensitivity analysis demonstrated the high degree of sensitivity of the TSEB model to the accuracy of the TR data, while the DATTUTDUT model was insensitive to systematic errors in TR as is the case with contextual-based models. However, it is shown that the study domain and spatial resolution will significantly influence the ET estimation from the DATTUTDUT model. Future work is planned for developing a hybrid approach that leverages the strengths of both modeling schemes and is simple enough to be used operationally with high-resolution imagery.
Highlights
As a key component of the land hydrological, energy, and biogeochemical cycles, evapotranspiration (ET) provides important information about terrestrial water availability and consumption (Evett et al, 2012)
High-resolution multispectral and thermal imagery obtained by aircraft mounted sensors were used to map evapotranspiration (ET) over two vineyards in central California using both the two-source energy balance (TSEB) and singlesource contextual-based DATTUTDUT (Deriving Atmosphere Turbulent Transport Useful To Dummies Using Temperature) model, which scales evaporative fraction (EF) between 0 and 1 using only the radiometric surface temperature (TR) extremes of cold/wet and hot/dry pixels in the remotely sensed scene
The TSEB model was able to derive satisfactory estimates of both instantaneous and daytime sensible heat flux (H ) and latent heat flux (LE) for all the five overpass dates, while overall the DATTUTDUT model output of H and LE were in less agreement with the tower measurements, for day of year (DOY) 162 and 219 overpass dates
Summary
As a key component of the land hydrological, energy, and biogeochemical cycles, evapotranspiration (ET) provides important information about terrestrial water availability and consumption (Evett et al, 2012). (In this paper we define satellite imagery with resolution on the order of ∼ 100 m as “medium resolution” and 1000 m as “moderate resolution” to distinguish from high-resolution imagery with meter-scale spatial resolution.) as water resources become more limited, there is a greater need for precision agricultural management at the field/subfield-scale, for high-valued or specialty crops (Zipper and Loheide II, 2014), and moderateresolution data are too coarse to inform variable rate application of water or nutrients within a field Obtaining both high spatial and temporal resolution data is not feasible with the current satellite constellation since medium resolution Earth observations have a long (2 or more weeks) revisit cycle, when considering cloud cover (Cammalleri et al, 2013)
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