John D. Valiantzas

Identification of the SCS-CN Parameter Spatial Distribution Using Rainfall-Runoff Data in Heterogeneous Watersheds

The Soil Conservation Service Curve Number (SCS-CN) method is widely used for predicting direct runoff volume for a given rainfall event. However, previous results indicated that when the CN value is determined from measured rainfall-runoff data in a natural watershed it is not possible to attribute a single CN value to the watershed, but actually the calculated CN values vary systematically with the rainfall depth. In a previous study, the authors investigated the hypothesis that the observed correlation between the calculated CN value and the rainfall depth in a watershed reflects the effect of the inevitable presence of soil-cover complex spatial variability along watersheds. In this study, a method to determine SCS-CN parameter values from rainfall-runoff data in heterogeneous watersheds is proposed. This method exploits the observed correlation between the calculated CN values and the rainfall depths in order to identify the spatial distribution of CN values along the watershed taking in to account the specific characteristics of the watershed. The proposed method utilizes the available rainfall-runoff data, remote sensing data and GIS techniques in order to provide information on spatial watershed characteristics that drive hydrological behavior. Furthermore, it allows the estimation of CN values for specific soil-land cover complexes in more complex watersheds. The proposed method was tested in a small experimental watershed in Greece. The watershed is equipped with a dense hydro-meteorological network, which together with a detailed land cover and soil survey using remote sensing and GIS techniques provided the detailed data required for this analysis.

Furrow infiltration estimation from time to a single advance point

Abstract A simple and quick method to determine the Soil Conservation Service (SCS) intake function in furrow irrigation is presented. The time of advance at only one location of the field, inflow rate, and average flow area are the only field data required to estimate the two parameters of the SCS infiltration equation. The dependence of the two intake parameters, k and α, of the SCS intake function was expressed analytically and then the single unknown intake parameter of the SCS function, α, could be determined by applying a volume–balance (VB) equation using a power advance assumption. Estimates of infiltration by the proposed method were compared with measured furrow infiltration data and a recently developed one-point method which uses the two parameter Philip infiltration equation, but is restricted by an assumption that the advance trajectory follows the power function with the exponent of 1/2. It is shown that the proposed one-point method can give more accurate results than the previous one-point method.

Simplified Reference Evapotranspiration Formula Using an Empirical Impact Factor for Penman’s Aerodynamic Term

AbstractRecently, a simple algebraic formula, equivalent in accuracy to the Penman equation, was derived for computing evapotranspiration from readily available measured data. The derivation of the formula was based on simplifications made to the standardized form of the Penman equation. In this paper, a weighted coefficient is introduced that accounts for different impacts of the aerodynamic component at two different ranges of relative humidity. The new formula for estimating reference evapotranspiration (ET0) is obtained by calibration using meteorological data from the CLIMWAT global database. The performance of the new derived formula was tested under various climatic conditions using daily data for 12 weather stations obtained from the CIMIS database. For many places where reliable wind speed data are not available, another expression was also suggested. An alternative method not requiring wind data was also tested: the reduced-set food and agriculture organization Penman-Monteith method, according ...

Combined Brooks-Corey/Burdine and van Genuchten/ Mualem Closed-Form Model for Improving Prediction of Unsaturated Conductivity

The performance of the widely used conventional closed-form models of Brooks and Corey and van Genuchten is restricted to soils characterized by a specific shape form of the water retention curve. Hence, the van Genuchten model more accurately describes “S”-shaped retention curves characterizing finer-textured soils, whereas the Brooks-Corey model is much better adapted for “J”-shaped retention curves characterizing relatively coarse-textured soils. In this work, a new closed-form soil hydraulic model is proposed. The suggested continuous-form function accurately describes soil retention curves irrespective of their specific shape form. New algebraic expressions based on Mualem’s statistical model and another new model that is a combination of the Mualem and Burdine theories were derived for the prediction of the unsaturated hydraulic conductivity function. Comparisons of ten soils known from international bibliography were performed. It is concluded that the proposed water retention curve expression as w...

Surface water storage independent equation for predicting furrow irrigation advance

Abstract A simple equation is developed to predict the advance rate of flow in furrows. The proposed equation does not use as inputs the data required for estimating the surface storage. In previous surface storage independent models it is generally assumed that the surface storage volume is negligible (compared with infiltrated volume). The proposed equation is derived by eliminating the surface storage term from the original volume balance equation and its derivative. The suggested equation thus needs no assumption about the magnitude of the value of surface storage volume. Infiltration is described by the extended Kostiakov-Lewis formula. The suggested equation is compared with observed furrow data, with the numerical kinematic-wave model and with a recently developed numerical model that ignores surface storage. For furrows in which the surface storage is not significant (compared with infiltration) all models predict advance reasonably well. For furrows in which the surface storage is relatively important, the proposed equation predicts advance with good accuracy, whereas previous models ignoring the surface storage greatly overpredict the advance rate.

Discussion of “Case Study on the Accuracy and Cost/Effectiveness in Simulating Reference Evapotranspiration in West-Central Florida” by Michael Grant Exner-Kittridge and Mark Cable Rains

John D. Valiantzas Professor, Div. of Water Resource Management, Dept. of Natural Resources and Agricultural Engineering, Agricultural Univ. of Athens, 75 Iera Odos, 11855, Athens, Greece. E-mail: lhyd2vay@aua.gr The authors evaluated various alternative reference ET equations based on various combinations of measured meteorological data for their accuracy in conjunction with their corresponding cost. The evaluated equations concern various reduced-data versions of the ASCE-PM standardized procedures (Allen et al. 2005), versions of the Priestley-Taylor equation, the Abtew (1996) radiation-type empirical formula, and the Hargreaves temperature-based equation. The various combinations of measured meteorological data examined are (Rs, G, U, RH, T); (Rs, G, T); (Rs, U, RH, T); (Rs, T); (T) (Rs, G, T); and (Rs). However, the authors did not examine the case where the only measured data are (Rs, RH, T). This combination of measured data approaches economically more closely the (Rs, T) and (Rs) combinations than the (Rs, U, RH, T) combination

Laboratory determination of unsaturated hydraulic conductivity using a generalized-form hydraulic model

Abstract The application of conventional pore structure models that predict the hydraulic-conductivity/watercontent function, K ( Θ ), from the soilwater retention data, yield K ( Θ ) expressions which in some cases deviate significantly from the actual K ( Θ ). An alternative method for the determination of K ( Θ ) is developed, in which a statistical model in a more general form with an additional unknown parameter is used, in conjunction with the one-step outflow laboratory method. Previous results were reformulated on the basis of the Brooks and Corey model to obtain the cumulative outflow data by semi-analytical formulae. The procedure suggested was applied in two different porous materials.

Rapid Graphical Detection of Weakness Problems in Numerical Simulation Infiltration Models Using a Linearized Form Equation

Recently, Valiantzas proposed a new two-parameter vertical infiltration equation that can be transformed to a linearized-form equation that essentially states that the shape of the cumulative infiltration data, when presented in the form of (i2/t) versus i, is linear. In this paper, the presentation of the numerical data to the Valiantzas linearized-form equation is proposed as an additional criterion to detect easily and rapidly possible errors of the numerical solutions and eventually to choose the best spatial discretization for a simulated infiltration event that is used as setup parameter to the numerical infiltration models. Numerical data and analytical solutions were used to validate the proposed method.

Modified Form of the Extended Kostiakov Equation Including Various Initial and Boundary Conditions

The extended Kostiakov equation is intensively used in surface irrigation applications. Traditionally, the extended Kostiakov infiltration formula is calibrated for specific field conditions. However, there is a dependence of the extended Kostiakov coefficients on both initial and boundary conditions. In this paper, a new simplified methodology is developed to account extended Kostiakov κ variation for these effects. The purely empirical extended Kostiakov equation is transformed to a form of a modified version of the classical Philip two-term equation. This modification relates a physical parameter, the soil sorptivity S, with the purely empirical coefficient κ of the extended Kostiakov formula. Then, the variation of the sorptivity S for various water levels and initial water contents is given theoretically by a simple algebraic equation. The proposed correction was compared with numerical infiltration data with varying initial (water content) and boundary conditions (ponding depth) for two contrasting soils. Results indicate that the corrected infiltration curves converge well with the simulated ones.

Isolation of a 1D Infiltration Time Interval under Ring Infiltrometers for Determining Sorptivity and Saturated Hydraulic Conductivity: Numerical, Theoretical, and Experimental Approach

AbstractA new method for the isolation of the time interval in which cumulative double ring infiltration data strictly represents the one-dimensional (1D) infiltration is proposed. The infiltration time necessary for the wetting front to reach the bottom edge of the cylinders can be determined graphically by visual observation of cumulative double ring infiltration data, for both inner and outer cylinder, expressed in the form of a recently proposed 1D linearized infiltration equation. Afterwards, sorptivity, S, and saturated hydraulic conductivity, Ks, can be estimated by fitting the same equation on the double ring cumulative infiltration data until the specific time was graphically found. After the theoretical analysis, the proposed method was tested using numerical double ring infiltration data [HYDRUS-2D (two-dimensional)/3D (three-dimensional)]. The numerical results indicated that the proposed method provides parameter estimates for S and Ks of acceptable accuracy for the four selected soils that c...