Faculty Publications
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Item Performance evaluation of reference evapotranspiration equations across a range of Indian climates(2006) Nandagiri, L.; Kovoor, G.M.Reference crop evapotranspiration (ET0) is a key variable in procedures established for estimation of evapotranspiration rates of agricultural crops. In recent years, there is growing evidence to show that the more physically based FAO-56 Penman-Monteith (PM) combination method yields consistently more accurate ET0 estimates across a wide range of climates and is being proposed as the sole method for ET0 computations. However, other methods continue to remain popular among Indian practitioners either because of traditional usage or because of their simpler input data requirements. In this study, we evaluated the performances of several ET0 methods in the major climate regimes of India with a view to quantify differences in ET0 estimates as influenced by climatic conditions and also to identify methods that yield results closest to the FAO-56 PM method. Performances of seven ET0 methods, representing temperature-based, radiation-based, pan evaporation-based, and combination-type equations, were compared with the FAO-56 PM method using historical climate data from four stations located one each in arid (Jodhpur), semiarid (Hyderabad), subhumid (Bangalore), and humid (Pattambi) climates of India. For each location, ET0 estimates by all the methods for assumed hypothetical grass reference crop were statistically compared using daily climate records extending over periods of 3-4 years. Comparisons were performed for daily and monthly computational time steps. Overall results while providing information on variations in FAO-56 PM ET0 values across climates also indicated climate-specific differences in ET0 estimates obtained by the various methods. Among the ET0 methods evaluated, the FAO-56 Hargreaves (temperature-based) method yielded ET0 estimates closest to the FAO-56 PM method both for daily and monthly time steps, in all climates except the humid one where the Turc (radiation-based) was best. Considering daily comparisons, the associated minimum standard errors of estimate (SEE) were 1.35, 0.78, 0.67, and 0.31 mm/day, for the arid, semiarid, subhumid, and humid locations, respectively. For monthly comparisons, minimum SEE values were smaller at 0.95, 0.59, 0.38, and 0.20 mm/day for arid, semiarid, subhumid, and humid locations, respectively. These results indicate that the choice of an alternative simpler equation in a particular climate on the basis of SEE is dictated by the time step adopted and also it appears that the simpler equations yield much smaller errors when monthly computations are made. In order to provide simple ET0 estimation tools for practitioners, linear regression equations for preferred FAO-56 PM ET0 estimates in terms of ET0 estimates by the simpler methods were developed and validated for each climate. A novel attempt was made to investigate the reasons for the climate-dependent success of the simpler alternative ET0 equations using multivariate factor analysis techniques. For each climate, datasets comprising FAO-56 PM ET0 estimates and the climatic variables were subject to factor analysis and the resulting rotated factor loadings were used to interpret the relative importance of climatic variables in explaining the observed variabilities in ET0 estimates. Results of factor analysis more or less conformed the results of the statistical comparisons and provided a statistical justification for the ranking of alternative methods based on performance indices. Factor analysis also indicated that windspeed appears to be an important variable in the arid climate, whereas sunshine hours appear to be more dominant in subhumid and humid climates. Temperature related variables appear to be the most crucial inputs required to obtain ET0 estimates comparable to those from the FAO-56 PM method across all the climates considered. © 2006 ASCE.Item Modelling soil moisture under different land covers in a sub-humid environment of Western Ghats, India(Indian Academy of Sciences, 2011) Venkatesh, B.; Nandagiri, L.; Purandara, B.K.; Reddy, V.B.The objective of this study is to apply and test a simple parametric water balance model for prediction of soil moisture regime in the presence of vegetation. The intention was to evaluate the differences in model parameterization and performance when applied to small watersheds under three different types of land covers (Acacia, degraded forest and natural forest). The watersheds selected for this purpose are located in the sub-humid climate within the Western Ghats, Karnataka, India. Model calibration and validation were performed using a dataset comprising depth-averaged soil moisture content measurements made at weekly time steps from October 2004 to December 2008. In addition to this, a sensitivity analysis was carried out with respect to the water-holding capacity of the soils with the aim of explaining the suitability and adaptation of exotic vegetation types under the prevailing climatic conditions. Results indicated reasonably good performance of the model in simulating the pattern and magnitude of weekly average soil moisture content in 150 cm deep soil layer under all three land covers. This study demonstrates that a simple, robust and parametrically parsimonious model is capable of simulating the temporal dynamics of soil moisture content under distinctly different land covers. Also, results of sensitivity analysis revealed that exotic plant species such as Acacia have adapted themselves effectively to the local climate. © Indian Academy of Sciences.Item Soil water fluxes under different land covers - A case study from Western Ghats, India(2011) Venkatesh, B.; Nandagiri, L.; Purandara, B.K.Knowledge of soil water fluxes is essential in hydrologic studies related to infiltration, runoff, ground water recharge and water uptake by vegetation. Previous studies have investigated the role played by soil and atmospheric factors on soil water fluxes in the unsaturated zone, but few studies have investigated the role played by vegetation or land cover. The present study was taken up understand the effect of land cover on soil water fluxes through long-term field measurements made in three experimental watersheds located in Western Ghats mountain ranges covering a portion of Uttara Kannada District, Karnataka State, India. Soil and climatic conditions were the same for the selected watersheds but they possessed different land covers - natural forest, degraded forest and acacia plantation. In addition to measurements of hydro-meteorological parameters, soil matric potential measurements were made at 4 locations in each of the watersheds up to a depth of 150 cm at an interval of 50 cm. Measurements were made for a period of 2 years (2007-2008) at weekly time intervals Depth-wise soil matric potential measurements were used to estimate soil water fluxes using Darcy's equation for unsaturated porous media. The estimated values of soil water fluxes were analyzed for their temporal distribution and stability. Results indicated that there is an improvement in soil moisture holding under the acacia plantation in comparison to degraded watershed. The estimated deviation of the soil water flux from the field average values indicate that the points located on milder slopes are representative of watershed mean soil water flux. Results also indicated the temporal persistence of soil water fluxes. © 2011 CAFET-INNOVA technical society. All right reserved.Item Quantifying aquifer properties and freshwater resource in coastal barriers: A hydrogeophysical approach applied at Sasihithlu (Karnataka state, India)(2012) Vouillamoz, J.-M.; Hoareau, J.; Grammare, M.; Caron, D.; Nandagiri, L.; Legchenko, A.Many human communities living in coastal areas in Africa and Asia rely on thin freshwater lenses for their domestic supply. Population growth together with change in rainfall patterns and sea level will probably impact these vulnerable groundwater resources. Spatial knowledge of the aquifer properties and creation of a groundwater model are required for achieving a sustainable management of the resource. This paper presents a ready-to-use methodology for estimating the key aquifer properties and the freshwater resource based on the joint use of two non-invasive geophysical tools together with common hydrological measurements.
We applied the proposed methodology in an unconfined aquifer of a coastal sandy barrier in South-Western India. We jointly used magnetic resonance and transient electromagnetic soundings and we monitored rainfall, groundwater level and groundwater electrical conductivity. The combined interpretation of geophysical and hydrological results allowed estimating the aquifer properties and mapping the freshwater lens. Depending on the location and season, we estimate the freshwater reserve to range between 400 and 700 L m??'2 of surface area (A± 50%). We also estimate the recharge using time lapse geophysical measurements with hydrological monitoring. After a rainy event close to 100% of the rain is reaching the water table, but the net recharge at the end of the monsoon is less than 10% of the rain. Thus, we conclude that a change in rainfall patterns will probably not impact the groundwater resource since most of the rain water recharging the aquifer is flowing towards the sea and the river. However, a change in sea level will impact both the groundwater reserve and net recharge. © Author(s) 2012.Item Vegetation dynamics in a tropical river basin inferred from MODIS satellite data(2013) Laxmi, K.; Nandagiri, L.The objective of this study was to analyze temporal and spatial dynamics of vegetation and land use/land cover (LU/LC) characteristics in a humid tropical river basin originating in the forested Western Ghats mountain ranges using the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. Both intra-annual and inter-annual variations in the parameters related to vegetation were analyzed in the Netravathi river basin (3314 km2) which is located in Karnataka State, India. MODIS data products on Land Surface Temperature and Reflectance were used as input to map the pixel-wise variations in albedo, Normalized Difference Vegetation Index (NDVI), Fraction of Vegetation (Fr) and Land Surface Temperature (LST) for two dates each (summer and winter) during the years 2002 and 2006. The fact that 2002 experienced a relatively wet summer followed by a relatively dry winter and 2006 experienced opposite conditions, proved useful in interpreting variations as influenced by wetness conditions. Overall results indicated significant variability in the parameters for major LU/LC classes of evergreen /semievergreen forest, scrub forest and agriculture. While albedo values appeared quite sensitive to wetness conditions, NDVI (and Fr) exhibited significant seasonal changes for some LU/LC classes but remained largely unaffected by wetness conditions. LST values corrected for elevation effects (LST*) were influenced by both LU/LC and wetness conditions. Differences in LST* values were as high as 70K between summer and winter of 2006 for some LU/LC classes. Lowest temperatures were recorded for the evergreen/ semievergreen forest class. Similar inferences could be drawn when variations in parameters were analyzed for 20 selected pixels located at different elevations and possessing each of the eight LU/LC classes. The methodology proposed in this research may prove to be useful in regional scale monitoring and mapping of tropical forests and other LU/LC categories in a convenient and cost-effective manner. MODIS satellite data products used in this study provides information on surface characteristics at a reasonable resolution. This permits identification of not only differences in LU/LC classes but also on changes in surface characteristics as influenced by season and wetness conditions. © 2013 CAFET-INNOVA TECHNICAL SOCIETY.Item Evaluating uncertainty of the soil and water assessment tool (SWAT) model in the upper cauvery basin, Karnataka, India(CAFET INNOVA Technical Society 1-2-18/103, Mohini Mansion, Gagan Mahal Road, Domalguda, Hyderabad 500029, 2015) Kumar Raju, B.C.; Nandagiri, L.Quantification of uncertainties associated with hydrological models are essential for accurate assessment of water balance components and optimal planning and management of water and land resources at basin-scale. The present study was taken up to evaluate the uncertainties associated with the Soil and Water Assessment Tool (SWAT) model using for two different techniques: i) Generalized Likelihood Uncertainty Estimation (GLUE) and ii) Sequential Uncertainty Fitting (SUFI-2) techniques. The study was carried out in the Upper Cauvery River basin (36,682 km2) located in the humid to sub-humid region of Karnataka State, India. The calibration of the model was carried out using the Nash – Sutcliffe (NS) coefficient as the objective function for both GLUE and SUFI-2 techniques. The P-factor was 67% and 71% of observed streamflow data bracketed by the 95% prediction uncertainty (95PPU) for GLUE and SUFI-2 respectively during calibration period and corresponding values of 54% and 61% during validation period. Overall results indicate the applicability of SWAT model with moderate levels of uncertainty in large basins located in the humid tropics. The calibrated SWAT model can be used for assessment of water balance components and land use management scenarios in the Upper Cauvery Basin. © 2015 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.Item Assessment of variable source area hydrological models in humid tropical watersheds(Taylor and Francis Ltd. michael.wagreich@univie.ac.at, 2018) Kumar Raju, B.C.K.; Nandagiri, L.The objective of this study was to compare the performances of hydrological models that incorporate the Variable Source Area (VSA) mechanism of runoff generation with that of the Soil and Water Assessment Tool (SWAT), which uses the infiltration-excess mechanism. One of the VSA-based model used, SWAT–VSA, has been proposed as a re-conceptualization of the SWAT and uses a topography-based wetness index to identify source areas. In this study, the topography-based wetness index was replaced with a Modified Normalized Difference Water Index (MNDWI) derived from satellite imagery resulting in the SWAT–MNDWI model. Model performances were evaluated through their application in two humid tropical watersheds (Hemavathi–2974 km2; Harangi–538.8 km2) located in the Upper Cauvery River Basin, India. Using relevant data inputs, the three models were applied separately to both watersheds. Models were calibrated for the historical period 2000–2003 and validated for the period 2004–2006 using observed daily observed streamflow records at the watershed outlets. Overall, the SWAT–MNDWI model was the best one in simulating daily streamflow with Nash–Sutcliffe efficiency of 0.85, coefficient of determination of 0.88, percentage bias of 13.2% and root mean square error of 37.48 m3/s for the Hemavathi watershed and corresponding values of 0.88, 0.88, 1.09% and 16.67 m3/s for the Harangi watershed. The spatial patterns of surface runoff generation were similar for the SWAT–VSA and SWAT–MNDWI models, but completely different for the SWAT model. Overall results have demonstrated that models incorporating VSA hydrology, and in particular the proposed SWAT–MNDWI model, provide accurate and convenient tools for distributed hydrologic modelling in humid tropical watersheds. © 2017 International Association for Hydro-Environment Engineering and Research.Item Extraction of Nonlinear Trends in Time Series of Rainfall Using Singular Spectrum Analysis(American Society of Civil Engineers (ASCE) onlinejls@asce.org, 2020) Aswathaiah, U.; Nandagiri, L.Characterization of nonlinear trends in time series of hydroclimatic variables exhibiting nonstationarity is necessary for more realistic projections of climate change and for optimal design of hydraulic structures. The present study was conducted to demonstrate the applicability of a novel Monte-Carlo-based singular spectrum analysis (SSA) to characterize nonlinear trends in historical time series of rainfall characteristics. Long-term (1960-2015) rainfall records for 17 gauges located in the Malaprabha River Basin, India, were used to analyze spatiotemporal variabilities of trends in rainfall totals and number of rainy days for annual and seasonal time periods. While the traditional Sen's Slope and Mann-Kendall (MK) trend tests indicated statistically nonsignificant decreasing monotonic trends at most gauge stations, SSA revealed the existence of steep nonlinear trends and distinct change points in the direction of the trend over the period of record for both rainfall and rainy days. Results of this study demonstrate the potential for SSA to extract crucial information on the trajectories of nonlinear trends and change points in time series of hydroclimatic variables that exhibit nonstationarity. © 2020 American Society of Civil Engineers.