8 research outputs found
Validation and downscaling of Advanced Scatterometer (ASCAT) soil moisture using ground measurements in the Western Cape, South Africa
No full textSatellite-based remote sensing of soil water content (SWC) is a promising technology for hydrological applications to overcome large spatiotemporal variabilities of SWC. This study investigated the performance of the Advanced Scatterometer (ASCAT) soil moisture product on METOP satellite (~12.5 km and downscaled to ~1 km resolution), against ground measurements of SWC taken with a Hydrosense II probe along transects of 360?820 m on agricultural and natural land at locations in the Western Cape. The ASCAT products estimated fairly accurately seasonal trends of SWC; performance was better on lower slopes (R 2 = 0.66) and uniform vegetation. ASCAT 12.5 km performed better in estimating SWC than the downscaled product (average concordance coefficient = 0.60 and 0.39, and R 2 = 0.84 and 0.74, respectively). ASCAT 12.5 km was more responsive to rainfall events, whilst the downscaled product was more sensitive to vegetation characteristics (normalised difference vegetation index and land surface temperature). In situations with ground measurement networks and data availability constraints, remote sensing could be a feasible alternative to monitor SWC for hydrological applications at the meso-scale (regional scale).Fil: Moller, Jason. University of the Western Cape; SudáfricaFil: Jovanovic, Nebo. CSIR Natural Resources and the Environment; Sudáfrica. University of the Western Cape; SudáfricaFil: García, César Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Católica de Córdoba; ArgentinaFil: Bugan, Richard D. H.. CSIR Natural Resources and the Environment; SudáfricaFil: Mazvimavi, Dominic. University of the Western Cape; Sudáfric
Estimación de la lluvia de diseño mediante información multisatélite de libre disponibilidad
No full textLa amenaza hídrica se cuantifica mediante el empleo de la Creciente de Proyecto. Si ésta no se puede derivar medianteestadística de caudales o niveles históricos, se evalúa partiendo de las relaciones intensidad de lluvia-duración-Recurrencia(i-d-T). Esta metodología es más común pero necesita de extensos registros históricos de lluvias, que en general no estándisponibles o son de difícil acceso. Una posible solución se puede entrever en las nuevas tecnologías satelitales que monitoreanla atmosfera y sus procesos. En este trabajo se desarrolla una metodología para generar una serie histórica de lámina máximadiaria, mediante el empleo de información multisatelital de libre disponibilidad derivada por el algoritmo RFE. Se trabajóen la cuenca experimental del río Sandspruit, Sudáfrica, que cuenta con cuatro series pluviométricas de 25 años de registroen los alrededores y series más cortas (3 años) dentro de la cuenca. La técnica propuesta permitió calcular la i-d-T en la zonade estudio con una resolución de pixel de casi 1km. Los mapas de precipitación resultantes para distintas recurrenciaspermitirán mejorar la exactitud de los cálculos de proyectos y por consiguiente optimizar la inversión en una obra civil.Fil: Catalini, Carlos Gastón. Instituto Nacional del Agua. Gerencia de Programas y Proyectos. Centro de la Region Semiarida.; ArgentinaFil: Garcia Rodriguez, Carlos Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Estudios Avanzados en Ingeniería y Tecnología. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto de Estudios Avanzados en Ingeniería y Tecnología; ArgentinaFil: García, César Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Instituto Nacional del Agua. Gerencia de Programas y Proyectos. Centro de la Region Semiarida.; ArgentinaFil: Jovanovic, Nebo. Council for Scientific and Industrial Research. Natural Resources And Environment, Stellenbosch; SudáfricaFil: Bugan, Richard. Council for Scientific and Industrial Research. Natural Resources And Environment, Stellenbosch; Sudáfric
Dynamics of MODIS evapotranspiration in South Africa
No full textThis paper describes the dynamics of evapotranspiration (ET) in South Africa using MOD16 ET satellite-derived data, and analyses the inter-dependency of variables used in the ET algorithm of Mu et al. (2011). Annual evapotranspiration is strongly dependent on rainfall and potential evapotranspiration (PET) in 4 climatically different regions of South Africa. Average ET in South Africa (2000–2012) was estimated to be 303 mm·a-1 or 481.4 x 109 m3·a1 (14% of PET and 67% of rainfall), mainly in the form of plant transpiration (T, 53%) and soil evaporation (Soil E, 39%). Evapotranspiration (ET) showed a slight tendency to decrease over the period 2000–2012 in all climatic regions, except in the south of the country (winter rainfall areas), although annual variations in ET resulted in the 13-year trends not being statistically significant. Evapotranspiration (ET) was spatially dependent on PET, T and vapour pressure deficit (VPD), in particular in winter rainfall and arid to semi-arid climatic regions. Assuming an average rainfall of 450 mm·a-1, and considering current best estimates of runoff (9% of rainfall), groundwater recharge (5%) and water withdrawal (2%), MOD16 ET estimates were about 15% short of the water balance closure in South Africa. The ET algorithm can be refined and tested for applications in restricted areas that are spatially heterogeneous and by accounting for soil water supply limiting conditions
Validation of remotely-sensed evapotranspiration and NDWI using ground measurements at Riverlands, South Africa
No full textQuantification of the water cycle components is key to managing water resources. Remote sensing techniques and products have recently been developed for the estimation of water balance variables. The objective of this study was to test the reliability of LandSAF (Land Surface Analyses Satellite Applications Facility) evapotranspiration (ET) and SPOT-Vegetation Normalised Difference Water Index (NDWI) by comparison with ground-based measurements. Evapotranspiration (both daily and 30 min) was successfully estimated with LandSAF products in a flat area dominated by fynbos vegetation (Riverlands, Western Cape) that was representative of the satellite image pixel at 3 km resolution. Correlation coefficients were 0.85 and 0.91 and linear regressions produced R2 of 0.72 and 0.75 for 30 min and daily ET, respectively. Ground-measurements of soil water content taken with capacitance sensors at 3 depths were related to NDWI obtained from 10-daily maximum value composites of SPOT-Vegetation images at a resolution of 1 km. Multiple regression models showed that NDWI relates well to soil water content after accounting for precipitation (adjusted R2 were 0.71, 0.59 and 0.54 for 10, 40 and 80 cm soil depth, respectively). Changes in NDWI trends in different land covers were detected in 14-year time series usin
Hydrogeological modelling of the Atlantis aquifer for management support to the Atlantis Water Supply Scheme
No full textHydrogeological modelling of the Atlantis aquifer for management support to the Atlantis Water Supply Scheme
No full textThe Atlantis Water Supply Scheme (AWSS, Western Cape, South Africa) has been in operation for about 40 years as a means to supply and augment drinking water to the town of Atlantis via managed aquifer recharge (MAR). In this study, the numerical model MODFLOW for groundwater flow and contaminant transport was used in support of the management of the AWSS. The aims were: (i) to calibrate the MODFLOW model for the MAR site at Atlantis; (ii) to run realistic scenarios that cannot be replicated through experiments; and (iii) to make recommendations in support of efficient and sustainable management of the aquifer. MODFLOW was calibrated through comparison of observed and simulated groundwater levels (R2 between 0.663 and 0.995). Scenario simulations indicated possible drawdowns between < 5 m (low groundwater abstraction and low artificial recharge of groundwater through infiltration basins) and > 20 m (high abstraction and high artificial recharge) at localized areas of the Witzand wellfield. At Silwerstroom, large drawdown levels were not predicted to occur, so this wellfield could be exploited more without affecting the sustainability of the groundwater resource. Groundwater moves from the infiltration basins towards the Witzand wellfield at a rate of 120–150 m·a-1. The modelling results supported recommendations for balancing groundwater abstraction and artificial recharge volumes, monitoring the water balance components of the system, the potential risks of groundwater contamination and the delineation of groundwater protection zones.Keywords: Groundwater abstraction; managed aquifer recharge; MODFLOW; particle tracking; scenario modellin
Four decades of water recycling in Atlantis (Western Cape, South Africa): Past, present and future
No full textThe primary aquifer at Atlantis (Western Cape, South Africa) is ideally suited for water supply and the indirect recycling of urban stormwater runoff and treated domestic wastewater for potable purposes. The relatively thin, sloping aquifer requires careful management of the artificial recharge and abstraction for balancing water levels. Water quality management is a further key issue at Atlantis for ensuring the highest quality potable water. Groundwater quality varies from point to point in the aquifer, while urban runoff and wastewater qualities vary greatly. The layout of the town allows for the separation of stormwater from the residential and industrial areas as well as separate treatment of domestic and industrial wastewater. This permits safe artificial recharge of the various water quality portions at different points in the aquifer, either for recycling or for preventing seawater intrusion. All of the management actions are dependent on detailed data collection and this paper describes the various parts of the system, describes the data collection activities, and provides results of the monitoring and aquifer responses over the past four decades. Challenges related to iron fouling of production boreholes are also described. The presence of emerging contaminants was studied in 2008 but requires follow-up research for establishing the extent of any possible threat to water recycling. In order to address the shortcomings of the system a risk management plan based on the Hazard Analysis and Critical Control Points approach was developed. Lessons learnt from the Atlantis experience can be transferred to other potential sites for establishment of similar systems in arid and semi-arid areas of South Africa and the African continent.Keywords: Atlantis, managed aquifer recharge, water recycling, groundwater, stormwater, wastewater, monitorin
Comparison of two remote sensing models for estimating evapotranspiration:algorithm evaluation and application in seasonally arid ecosystems in South Africa
No full textRemote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration (ET) models in recent years, important information gaps still exist on the accuracy of the models particularly in arid and semi-arid environments. In this study, we evaluated the Penman-Monteith based MOD16 and the modified Priestley-Taylor (PT-JPL) models at the daily time step against three measured ET datasets. We used data from two summer and one winter rainfall sites in South Africa. One site was dominated by native broad leaf and the other by fine leafed deciduous savanna tree species and C4 grasses. The third site was in the winter rainfall Cape region and had shrubby fynbos vegetation. Actual ET was measured using open-path eddy covariance systems at the summer rainfall sites while a surface energy balance system utilizing the large aperture boundary layer scintillometer was used in the Cape. Model performance varied between sites and between years with the worst estimates (R2<0.50 and RMSE>0.80 mm/d) observed during years with prolonged mid-summer dry spells in the summer rainfall areas. Sensitivity tests on MOD16 showed that the leaf area index, surface conductance and radiation budget parameters had the largest effect on simulated ET. MOD16 ET predictions were improved by: (1) reformulating the emissivity expressions in the net radiation equation; (2) incorporating representative surface conductance values; and (3) including a soil moisture stress function in the transpiration sub-model. Implementing these changes increased the accuracy of MOD16 daily ET predictions at all sites. However, similar adjustments to the PT-JPL model yielded minimal improvements. We conclude that the MOD16 ET model has the potential to accurately predict water use in arid environments provided soil water stress and accurate biome-specific parameters are incorporated.</p
