M. Sekhar

Retrieval and Multi-scale Validation of Soil Moisture from Multi-temporal SAR Data in a Semi-Arid Tropical Region

The current study presents an algorithm to retrieve surface Soil Moisture (SM) from multi-temporal Synthetic Aperture Radar (SAR) data. The developed algorithm is based on the Cumulative Density Function (CDF) transformation of multi-temporal RADARSAT-2 backscatter coefficient (BC) to obtain relative SM values, and then converts relative SM values into absolute SM values using soil information. The algorithm is tested in a semi-arid tropical region in South India using 30 satellite images of RADARSAT-2, SMOS L2 SM products, and 1262 SM field measurements in 50 plots spanning over 4 years. The validation with the field data showed the ability of the developed algorithm to retrieve SM with RMSE ranging from 0.02 to 0.06 m(3)/m(3) for the majority of plots. Comparison with the SMOS SM showed a good temporal behaviour with RMSE of approximately 0.05 m(3)/m(3) and a correlation coefficient of approximately 0.9. The developed model is compared and found to be better than the change detection and delta index model. The approach does not require calibration of any parameter to obtain relative SM and hence can easily be extended to any region having time series of SAR data available.

Modelling flow through unsaturated zones: Sensitivity to unsaturated soil properties

A numerical model to simulate moisture flow through unsaturated zones is developed using the finite element method, and is validated by comparing the model results with those available in the literature. The sensitivities of different processes such as gravity drainage and infiltration to the variations in the unsaturated soil properties are studied by varying the unsaturated parameters α andn over a wide range. The model is also applied to predict moisture contents during a field internal drainage test.

Temporal Moments for Reactive Transport through Fractured Impermeable/Permeable Formations

AbstractThe transport of reactive solutes through fractured porous formations has been analyzed. The transport through the porous block is represented by a general multiprocess nonequilibrium equation (MPNE), which, for the fracture, is represented by an advection-dispersion equation with linear equilibrium sorption and first-order transformation. An implicit finite-difference technique has been used to solve the two coupled equations. The transport characteristics have been analyzed in terms of zeroth, first, and second temporal moments of the solute in the fracture. The solute behavior for fractured impermeable and fractured permeable formations are first compared and the effects of various fracture and matrix transport parameters are analyzed. Subsequently, the transport through a fractured permeable formation is analyzed to ascertain the effect of equilibrium sorption, rate-limited sorption, and the multiprocess nonequilibrium transport process. It was found that the temporal moments were nearly ident...

Parameter estimation in an anisotropic leaky aquifer system

Abstract Parameter estimation is made for an anisotropic leaky aquifer system in which the direction of principal axes is unknown. There is a declining water table in the aquitard. The seven governing parameters are the direction of principal axes, the degree of anisotropy, the equivalent transmissivity and storage coefficient of the aquifer, and the leakage coefficient, specific storage and specific yield of the unconfined aquitard. A modified parameter perturbation technique which is computationally efficient is used for the determination of sensitivity coefficients. The parameter estimation procedure is applied to three test problems including one field problem. Situations where the total number of wells is only three are handled with two pump tests by locating the pump in a different well in each test.

MAPSM: A Spatio-Temporal Algorithm for Merging Soil Moisture from Active and Passive Microwave Remote Sensing

Availability of soil moisture observations at a high spatial and temporal resolution is a prerequisite for various hydrological, agricultural and meteorological applications. In the current study, a novel algorithm for merging soil moisture from active microwave (SAR) and passive microwave is presented. The MAPSM algorithm—Merge Active and Passive microwave Soil Moisture—uses a spatio-temporal approach based on the concept of the Water Change Capacity (WCC) which represents the amplitude and direction of change in the soil moisture at the fine spatial resolution. The algorithm is applied and validated during a period of 3 years spanning from 2010 to 2013 over the Berambadi watershed which is located in a semi-arid tropical region in the Karnataka state of south India. Passive microwave products are provided from ESA Level 2 soil moisture products derived from Soil Moisture and Ocean Salinity (SMOS) satellite (3 days temporal resolution and 40 km nominal spatial resolution). Active microwave are based on soil moisture retrievals from 30 images of RADARSAT-2 data (24 days temporal resolution and 20 m spatial resolution). The results show that MAPSM is able to provide a good estimate of soil moisture at a spatial resolution of 500 m with an RMSE of 0.025 m3/m3 and 0.069 m3/m3 when comparing it to soil moisture from RADARSAT-2 and in-situ measurements, respectively. The use of Sentinel-1 and RISAT products in MAPSM algorithm is envisioned over other areas where high number of revisits is available. This will need an update of the algorithm to take into account the angle sampling and resolution of Sentinel-1 and RISAT data.

Simulated water resource impacts and livelihood implications of stakeholder-developed scenarios in the Jaldhaka Basin, India

This paper shows how multidisciplinary research can help policy makers develop policies for sustainable agricultural water management interventions by supporting a dialogue between government departments that are in charge of different aspects of agricultural development. In the Jaldhaka Basin in West Bengal, India, a stakeholder dialogue helped identify potential water resource impacts and livelihood implications of an agricultural water management rural electrification scenario. Hydrologic modelling demonstrated that the expansion of irrigation is possible with only a localized effect on groundwater levels, but cascading effects such as declining soil fertility and negative impacts from agrochemicals will need to be addressed.

Disaggregation of LST over India: comparative analysis of different vegetation indices

ABSTRACT The non-availability of high-spatial-resolution thermal data from satellites on a consistent basis led to the development of different models for sharpening coarse-spatial-resolution thermal data. Thermal sharpening models that are based on the relationship between land-surface temperature (LST) and a vegetation index (VI) such as the normalized difference vegetation index (NDVI) or fraction vegetation cover (FVC) have gained much attention due to their simplicity, physical basis, and operational capability. However, there are hardly any studies in the literature examining comprehensively various VIs apart from NDVI and FVC, which may be better suited for thermal sharpening over agricultural and natural landscapes. The aim of this study is to compare the relative performance of five different VIs, namely NDVI, FVC, the normalized difference water index (NDWI), soil adjusted vegetation index (SAVI), and modified soil adjusted vegetation index (MSAVI), for thermal sharpening using the DisTrad thermal sharpening model over agricultural and natural landscapes in India. Multi-temporal LST data from Landsat-7 Enhanced Thematic Mapper Plus (ETM+) and Moderate Resolution Imaging Spectroradiometer (MODIS) sensors obtained over two different agro-climatic grids in India were disaggregated from 960 m to 120 m spatial resolution. The sharpened LST was compared with the reference LST estimated from the Landsat data at 120 m spatial resolution. In addition to this, MODIS LST was disaggregated from 960 m to 480 m and compared with ground measurements at five sites in India. It was found that NDVI and FVC performed better only under wet conditions, whereas under drier conditions, the performance of NDWI was superior to other indices and produced accurate results. SAVI and MSAVI always produced poorer results compared with NDVI/FVC and NDWI for wet and dry cases, respectively.

Controls of DSi in streams and reservoirs along the Kaveri River, South India

There is an increasing body of evidence showing that land use may affect the concentration and flux of dissolved silica (DSi) and amorphous, biogenic Si particles (ASi/BSi) in surface waters. Here, we present a study of riverine waters collected within the Kaveri River Basin, which has a long history of land occupation with +43% population increase in the watershed during the last 30 years associated with agricultural practices including canal irrigation from reservoirs and, more recently, bore well pumping. We report total dissolved solids (TDS) and suspended material (TSM) for 15 river stations and 5 reservoirs along the Kaveri itself and its main tributaries sampled during pre-monsoon, monsoon and post-monsoon periods in 2006 and 2007. The TDS in the Kaveri River globally increases from the upper reaches (humid to sub-humid climate) to the lower reaches (semi-arid climate), and at a given station from monsoon (M) to hot season (HS). The DSi concentrations range from 129 μmol L(-1) (M) to 390 μmol L(-1) (HS) in the main Kaveri stream and reaches up to 686 μmol L(-1) in the Shimsha River (HS). Our results indicate that DSi and the main solutes of the Kaveri River have not drastically changed since the last 30 years despite the population increase. The pollution index of Van der Weijden and Pacheco (2006) ranges from 13% to 54% but DSi does not seem to be affected by domestic wastewater. ASi is mostly composed of diatoms and phytoliths that both play roles in controlling DSi. We suggest that DSi and ASi delivered to the cultivated areas through irrigation from reservoir may have two important consequences: increasing Si bioavailability for crops and limiting Si flux to the ocean.

Parameter estimation in an aquifer-water table aquitard system

Abstract The weighted least squares approach is used for the estimation of parameters in an aquifer-water table aquitard system. Six parameters are to be evaluated, namely: the equivalent transmissivity, degree of anisotropy and storage coefficient of the aquifer, and the leakage coefficient, specific storage and specificv yield of the aquitard. The coupled aquifer-aquitard equations are solved by an iterative numerical procedure and the optimisation problem is solved by the sensitivity analysis technique. This method is applied to one hypothetical problem and two field pumping tests of 7 days duration.

Analysis of an Aquifer-water table aquitard system

The problem of pumping an aquifer in an aquifer-water table aquitard system is considered, accounting for the elastic properties of both the aquifer and the aquitard, the gravity drainage in the aquitard and treating the water table as an unknown boundary. The coupled partial differential equations are nondimensionalised, yielding three principal parameters governing the problem. The numerical solution of these equations is obtained for a wide range of parameter values. Type curves are generated and their use is illustrated through a field application.