Rama Prasad

A linear root water uptake model

A model of root water extraction is proposed, in which a linear variation of extraction rate with depth is assumed. Five crops are chosen for simulation studies of the model, and soil moisture depletion under optimal conditions from different layers for each crop is calculated. Similar calculations are also made using the constant extraction rate model. Rooting depth is assumed to vary linearly with potential evapotranspiration for each crop during the vegetative phase. The calculated depletion patterns are compared with measured mean depletion patterns for each crop. It is shown that the constant extraction rate model results in large errors in the prediction of soil moisture depletion, while the proposed linear extraction rate model gives satisfactory results. Hypothetical depletion patterns predicted by the model in combination with a moisture tension-dependent sink term developed elsewhere are indicated.

Runoff processes in headwater catchments - an experimental study in Western Ghats, South India.

Field observations and experiments were conducted to study runoff in a headwater region in the mountains near the west coast of Karnataka, a south Indian state. Three catchments were studied. Pipeflow was observed in all three cases, and there was no subsurface lateral flow through the soil matrix. Pipeflow was a significant contributor to quickflow in two catchments. In the third, grassed catchment, a new mechanism of runoff, called pipe overland flow in this paper, was discovered, which contributed to quickflow. The contributing area for pipeflow increases with the magnitude as well as duration of rainfall, but a dual behaviour with respect to average rainfall intensity shows up in one of the catchments.

Understanding runoff processes using a watershed model - a case study in the Western Ghats in South India.

The wet tropical Western Ghat Mountain ranges in South India present an interesting combination of meteorological and physical characteristics. The results of a watershed model analysis carried out to understand the catchment response and the relative importance of different runoff processes in the region are reported in this paper. A lumped parameter model simulating saturated source area runoff, lateral flow through pipes and the saturated zone groundwater flow, has been developed assuming that source area runoff is the only quickflow component. The model has been calibrated on seven catchments using sufficiently long records of daily data. A wide range of tests has been used to show that the model performs reliably. The influence of catchment characteristics on the relative importance of the flow components and the catchment response has been studied. The model simulations have been interpreted to infer that the pipeflow contributions augment the contributions of source area runoff to stream quickflow. Suggestions for further research in the area are given, based on the inferences drawn.

Centrifugal pump transient characteristics and analysis using the method of characteristics

A theoretical and experimental study has been carried out on the transient characteristics of a centrifugal pump during starting and stopping periods. Experiments have been conducted on a volute pump with different valve openings to study the dynamic behaviour of the pump during normal start up and stopping, when a small length of discharge pipe line is connected to discharge flange of the pump. Similar experiments have also been conducted when the test pump was part of a hydraulic system to study the system effect on the transient characteristics. Instantaneous rotational speed, flowrate, and delivery and suction pressures of the pump are recorded and it is observed in ail the tested cases that the change of pump behaviour during the transient period is quasi-steady. The dynamic characteristics of the pump have been analysed by a numerical model using the method of characteristics. The model is presented and the results are compared with the experimental data. As the model contains speed acceleration and unsteady discharge terms, the model can be applied for analyses of purely unsteady cases where the pump dynamic characteristics show considerable departure from their steady-state characteristics.

Field evaluation of unsaturated hydraulic conductivity models and parameter estimation from retention data

Predictions of two popular closed-form models for unsaturated hydraulic conductivity (K) are compared with in situ measurements made in a sandy loam field soil. Whereas the Van Genuchten model estimates were very close to field measured values, the Brooks-Corey model predictions were higher by about one order of magnitude in the wetter range. Estimation of parameters of the Van Genuchten soil moisture characteristic (SMC) equation, however, involves the use of non-linear regression techniques. The Brooks-Corey SMC equation has the advantage of being amenable to application of linear regression techniques for estimation of its parameters from retention data. A conversion technique, whereby known Brooks-Corey model parameters may be converted into Van Genuchten model parameters, is formulated. The proposed conversion algorithm may be used to obtain the parameters of the preferred Van Genuchten model from in situ retention data, without the use of non-linear regression techniques.

Influence of time step in the simulation modelling of evapotranspiration

Three simulations of evapotranspiration were done with two values of time step,viz 10 min and one day. Inputs to the model were weather data, including directly measured upward and downward radiation, and soil characteristics. Three soils were used for each simulation. Analysis of the results shows that the time step has a direct influence on the prediction of potential evapotranspiration, but a complex interaction of this effect with the soil moisture characteristic, rate of increase of ground cover and bare soil evaporation determines the actual transpiration predicted. The results indicate that as small a time step as possible should be used in the simulation.

Detection of moisture stress by indicator plots — A simulation

The possibility of advanced indication of moisture stress in a crop by small prepared plots with compacted or partially sand-substituted soils is examined by an analytical simulation. A series of soils and three crops are considered for the simulation. The moisture characteristics of the soils are calculated with an available model. Using average potential evapotranspiration values and a simple actual evapotranspiration model, the onset of moisture stress in the natural and indicator plots is calculated for different degrees of sand substitution and compaction. Cases where sand substitution fails are determined. The effect of intervening rainfall and limited root depth on the beginning of moisture stress is investigated.

Analysis of Flood Frequencies in the Cauvery Valley

Annual Peak flood records from ten subbasins in the upper reaches of the river Cauvery in southIndia were analysed. It was found That floods of longer return periods were correlated well with the drainage area of the subbasin. While the correlation for floods of Short return periods was poor. Correlation with normal annual rainfall in the subbasin was also found to be poor. Return periods for design floods based on empirical formulae, used in the region for small reservoirs (called ‘tanks’). were calculated using the fitted distributions. These are much too high in relation to the expected life of the tanks in all cases but one. This would suggest that while the majority to the tank Spillways were overdesigned, some would be underdesigned. This conclusion is also supported by the fact that breach due to inadequate spillway Capacity is rare.

An information system for village handpumps

The need for a computerised information system for handpumps installed in any state is brought out. A system with organisational and software aspects is presented. The organisational aspect covers the generation and transmission of data on handpumps at the users’ and the government level. The software consists of a Fortran program, data files, an updating program and a transaction file. The periodic updating of data files is discussed. The system output covers many items of vital information needed for formulating maintenance and research and development policies. A test run of the program with an arbitrarily selected data base is discussed.