ALBERT

Description: There is increasing interest in using Gravity Recovery and Climate Experiment (GRACE) satellite data to remotely monitor groundwater storage variations; however, comparisons with ground-based well data are limited but necessary to validate satellite data processing, especially when the study area is close to or below the GRACE footprint. The Central Valley is a heavily irrigated region with large-scale groundwater depletion during droughts. Here we compare updated estimates of groundwater storage changes in the California Central Valley using GRACE satellites with storage changes from groundwater level data. A new processing approach was applied that optimally uses available GRACE and water balance component data to extract changes in groundwater storage. GRACE satellites show that groundwater depletion totaled ∼31.0 ± 3.0 km3 for Groupe de Recherche de Geodesie Spatiale (GRGS) satellite data during the drought from October 2006 through March 2010. Groundwater storage changes from GRACE agreed with those from well data for the overlap period (April 2006 through September 2009) (27 km3 for both). General correspondence between GRACE and groundwater level data validates the methodology and increases confidence in use of GRACE satellites to monitor groundwater storage changes.

Description: A tracer test was performed at the Rifle Integrated Field Research Challenge site to assess the effect of addition of bicarbonate on U(VI) desorption from contaminated sediments in the aquifer and to compare equilibrium and rate-limited reactive transport model descriptions of mass transfer limitations on desorption. The tracer test consisted of injection of a 37 mM NaHCO3 solution containing conservative tracers followed by down-gradient sampling of groundwater at various elevations and distances from the point of injection. Breakthrough curves show that dissolved U(VI) concentrations increased 1.2–2.6-fold above background levels, resulting from increases in bicarbonate alkalinity (from injectate solution) and Ca concentrations (from cation exchange). In general, more U(VI) was mobilized in shallower zones of the aquifer, where finer-grained sediments and higher solid phase U content were found compared to deeper zones. An equilibrium-based reactive transport model incorporating a laboratory-based surface complexation model derived from the same location predicted the general trends in dissolved U(VI) during the tracer test but greatly overpredicted the concentrations of U(VI), indicating that the system was not at equilibrium. Inclusion of a multirate mass transfer model successfully simulated the nonequilibrium desorption behavior of U(VI). Local sediment properties such as sediment texture (weight percent

Description: The surface energy balance algorithm for land (SEBAL) has been designed and widely used (and misused) worldwide to estimate evapotranspiration across varying spatial and temporal scales using satellite remote sensing over the past 15 yr. It is, however, beset by visual identification of a hot and cold pixel to determine the temperature difference (dT) between the surface and the lower atmosphere, which is assumed to be linearly correlated with surface radiative temperature (Trad) throughout a scene. To reduce ambiguity in flux estimation by SEBAL due to the subjectivity in extreme pixel selection, this study first demonstrates that SEBAL is of a rectangular framework of the contextual relationship between vegetation fraction (fc) and Trad, which can distort the spatial distribution of heat flux retrievals to varying degrees. End members of SEBAL were replaced by a trapezoidal framework of the fc-Trad space in the modified surface energy balance algorithm for land (M-SEBAL). The warm edge of the trapezoidal framework is determined by analytically deriving temperatures of the bare surface with the largest water stress and the fully vegetated surface with the largest water stress implicit in both energy balance and radiation budget equations. Areally averaged air temperature (Ta) across a study site is taken to be the cold edge of the trapezoidal framework. Coefficients of the linear relationship between dT and Trad can vary with fc but are assumed essentially invariant for the same fc or within the same fc class in M-SEBAL. SEBAL and M-SEBAL are applied to the soil moisture-atmosphere coupling experiment (SMACEX) site in central Iowa, U.S. Results show that M-SEBAL is capable of reproducing latent heat flux in terms of an overall root-mean-square difference of 41.1 W m−2 and mean absolute percentage difference of 8.9% with reference to eddy covariance tower-based measurements for three landsat thematic mapper/enhanced thematic mapper plus imagery acquisition dates in 2002. The retrieval accuracy of SEBAL is generally lower than M-SEBAL, depending largely on the selected extremes. Spatial distributions of heat flux retrievals from SEBAL are distorted to a certain degree due to its intrinsic rectangular framework.

Description: A key challenge in managing semiarid basins, such as in the Murray-Darling in Australia, is to balance the trade-offs between the net benefits of allocating water for irrigated agriculture, and other uses, versus the costs of reduced surface flows for the environment. Typically, water planners do not have the tools to optimally and dynamically allocate water among competing uses. We address this problem by developing a general stochastic, dynamic programming model with four state variables (the drought status, the current weather, weather correlation, and current storage) and two controls (environmental release and irrigation allocation) to optimally allocate water between extractions and in situ uses. The model is calibrated to Australia's Murray River that generates: (1) a robust qualitative result that “pulse” or artificial flood events are an optimal way to deliver environmental flows over and above conveyance of base flows; (2) from 2001 to 2009 a water reallocation that would have given less to irrigated agriculture and more to environmental flows would have generated between half a billion and over 3 billion U.S. dollars in overall economic benefits; and (3) water markets increase optimal environmental releases by reducing the losses associated with reduced water diversions.

Description: Proliferation of evapotranspiration (ET) products warrants comparison of these products. The study objective was to assess uncertainty in ET output from four land surface models (LSMs), Noah, Mosaic, VIC, and SAC in NLDAS-2, two remote sensing-based products, MODIS and AVHRR, and GRACE-inferred ET from a water budget with precipitation from PRISM, monitored runoff, and total water storage change (TWSC) from GRACE satellites. The three cornered hat method, which does not require a priori knowledge of the true ET value, was used to estimate ET uncertainties. In addition, TWSC or total water storage anomaly (TWSA) from GRACE was compared with water budget estimates of TWSC from a flux-based approach or TWSA from a storage-based approach. The analyses were conducted using data from three regions (humid – arid) in the South Central US as case studies. Uncertainties in ET are lowest in LSM ET (~5 mm/month), moderate in MODIS- or AVHRR-based ET (10 – 15 mm/month), and highest in GRACE-inferred ET (20 – 30 mm/month). There is a tradeoff between spatial resolution and uncertainty, with lower uncertainty in the coarser-resolution LSM ET (~14 km) relative to higher uncertainty in the finer-resolution (~ 1 ‒ 8 km) RS ET. Root-mean-square (RMS) of uncertainties in water budget estimates of TWSC is about half of RMS of uncertainties in GRACE-derived TWSC for each of the regions. Future ET estimation should consider a hybrid approach that integrates strengths of LSMs and satellite-based products to constrain uncertainties.

Description: This study examines the impact of endmember (i.e., hot and cold extremes) selection on the performance and mechanisms of error propagation in satellite-based spatial variability models for estimating actual evapotranspiration, using the triangle, Surface Energy Balance Algorithm for Land (SEBAL), and Mapping Evapotranspiration with high Resolution and Internalized Calibration (METRIC) models. These models were applied to the Soil Moisture-Atmosphere Coupling Experiment site in central Iowa on two Landsat Thematic Mapper/Enhanced Thematic Mapper Plus acquisition dates in 2002. Evaporative fraction (EF, defined as the ratio of latent heat flux to availability energy) estimates from the three models at field and watershed scales were examined using varying endmembers. Results show that the endmembers fundamentally determine the magnitudes of EF retrievals at both field and watershed scales. The hot and cold extremes exercise a similar impact on the discrepancy between the EF estimates and the ground-based measurements, i.e., given a hot (cold) extreme, the EF estimates tend to increase with increasing temperature of cold (hot) extreme, and decrease with decreasing temperature of cold (hot) extreme. The coefficient of determination between the EF estimates and the ground-based measurements depends principally on the capability of remotely sensed surface temperature ( T s ) to capture EF (i.e., depending on the correlation between T s and EF measurements), being slightly influenced by the endmembers. Varying the endmembers does not substantially affect the standard deviation and skewness of the EF frequency distributions from the same model at the watershed scale. However, different models generate markedly different EF frequency distributions due to differing model physics, especially the limiting edges of EF defined in the remotely sensed vegetation fraction ( f c ) and T s space. In general, the endmembers cannot be properly determined because: (1) they do not necessarily exist within a scene, varying with the spatial extent, resolution, and quality of satellite images being used; and/or (2) different operators can select different endmembers. Furthermore, the limiting edge of EF=0 in the f c - T s space varies with the model, with SEBAL-type models having inherently an increasing curvilinear limiting edge of EF=0 with f c . The spatial variability models therefore require careful calibration in order to deduce reasonable EF limiting edges and then confine the magnitudes of EF estimates.