P. Kerkides

New empirical formula for hourly estimations of reference evapotranspiration

Abstract A new empirical equation for estimating hourly reference evapotranspiration ET0 is proposed. This equation requires data for three pertinent meteorological attributes, solar radiation Rs, air temperature T, and relative humidity RH. A routine surface polynomial regression analysis in three stages, is employed in order to estimate the factors ci entering the empirical model ET 0 =f(R s , RH, T; ci). For the calibration of the proposed model, data sets collected from Copais (Greece) were used. Verification of the validity of the model was obtained using independent data from Copais as well as data from CIMIS (Davis, Sacramento, CA). A comparative evaluation of the model was performed against some of the most widely used and strongly recommended models for estimating hourly ET0. Among them are the recently proposed Penman–Monteith (FAO56-Penman–Monteith), CIMIS version of Penman (CIMIS-Penman), and the American Society of Civil Engineers version of Penman–Monteith (ASCE-PM). Statistics and scatterplots, using ASCE-PM as the standard model indicate that the new empirical equation (Copais) operates quite satisfactorily for both regions, therefore may provide the premises to become a tool, for routine hourly reference evapotranspiration calculations.

Water balance estimates over Greece

Abstract Water balance for 31 locations in Greece is calculated on the basis of long-term average monthly precipitation, evapotranspiration and combined soil and vegetation characteristics, according to the method proposed by Thomthwaite and Mather. Monthly evapotranspiration estimates are calculated from 27 years (1960–1987) of routine meteorological data using the original Penman method. Soil and vegetation characteristics specific for the locations under study are combined in the water capacity of the root zone (WCRZ). similar water balance calculations were carried out using various fixed values of WCRZ for all stations, to evaluate the effects of soil and vegetation through the WCRZ in the final estimates of soil moisture deficits. Water balance calculations were also performed using average monthly evapotranspiration estimates calculated according to the empirical Tbomthwaite method. Results were compared in order to show possible differences that could be attributed to the method of estimating evapotranspiration. Finally, results obtained with a value of WCRZ fixed at 300 mm and potential evapotranspiration estimated by the Thomthwaite method for the period 1969–1987 were compared with existing similar results over a longer period in the past (1931–1968), in order to detect diachronic changes in the water balance components over the same regions in Greece.

Semi-empirical approach for estimating actual evapotranspiration in Greece

Abstract Reliable estimation of actual evapotranspiration rates (ET) may be achieved if extensive information on the soil–plant–atmosphere system is available. Agricultural and irrigation engineers facing the problem of rational irrigation planning, rarely have at their disposal such information. Therefore, there is a demand for simpler approaches to estimate actual evapotranspiration. In this paper, a semi-empirical approach is proposed for estimating actual water losses from crops. It is assumed that the ratio of actual to maximum evapotranspiration (ET/ET m ) is an exponential function of the water content w in the root-zone, of the form: ET/ET m =exp[ c ( w − w fc )/( w − w wp )], where c is a constant introduced to adjust the decrease of the ratio ET/ET m according to what is observed for the climatic conditions, soils and crops of Greece, w fc the water content at field capacity and w wp denotes the water content at wilting point. Verification of the above approach for estimating actual evapotranspiration was achieved by comparing ET-values obtained by the soil moisture profile changes and the ET-values obtained by the above equation. Meteorological, crop and soil data required were collected from experimental fields of the Agricultural University of Athens (38°23′N, 23°6′E). The agreement of actual versus computed ET-values for three widely grown crops in Greece (cotton, wheat and maize) may be considered as satisfactory.

An investigation of the relationship between ponded and constant flux rainfall infiltration

The physics of the rainfall infiltration under a constant rainfall flux greater than the hydraulic conductivity at saturation are investigated and compared with those pertaining to the infiltration process under flooding. From the arguments developed theoretically the following conclusions are reached: The moisture profile prevailing at the time the soil surface becomes saturated during the rainfall process (t = T) is identical with the profile which has been developed during flooding at an earlier time at which the infiltration rate is equal to the rainfall flux (t = tc < T). The rainfall profile development after T is the same as that of the profile under flooding after tc, the two profiles having a constant time lag equal to (T−tc). The Green and Ampt analysis as has been applied to the constant flux rainfall infiltration presupposes the identity of the profiles mentioned above. In addition to the theoretical arguments, the flow equation was solved numerically for four different soils under the conditions that define the two infiltration processes. The numerical results obtained conform with the theory.

Estimation of the Water Resources Potential in the Island System of the Aegean Archipelago, Greece

Abstract The present paper tries to estimate the surface water resources potential in some of the major Aegean islands in an effort to provide a means for the continuous development of the region, and, by extension, for similar areas around the world. The islands have to confront the challenge of surviving in a semiarid environment under the constraints of uneven water resources distribution both in space and time. In addition to these, tourism development, industrialization and highly water consumptive life styles have exacerbated perennial problems in water resources and water resources management. The framework of the present effort has a two-prong emphasis. In the first part, a simulation model is presented, which tries to estimate the potential surface runoff under physical, structural organizational, and institutional constraints. The methodology and the premises of the simulation process are delineated. In the second part, the results of the models application in distinct cases are demarcated. The final product, namely the model and the resulting runoff coefficients, are presented in the form of a standard, which may provide practitioners in the field as well as decisionmakers the means for an initial reference in pertinent developmental efforts. Finally, the conclusions and recommendations raise the question of ecosystem resilience and point towards the urgent and continuous need for the application of integrated water resources management principles.

A Note to the Variable Sorptivity Infiltration Equation

A simplification for the variable sorptivity infiltration equation of Poulovassilis et al. (1989) is proposed. The resulting equation has three parameters Sx, c and K0. From these, Sx and c are considered as fitting parameters and K0 as a physical one. The new empirical infiltration equation is tested for precision, parameter time-dependence and applicability for soil surveys. The test was carried out by comparison with reference solutions i.e. infiltration data obtainedexperimentally, analytically or numerically for two different head conditionsat the infiltration surface. A good agreement is observed for all examinedcases. The dependence of the fitting parameters Sx and c on the initialand boundary conditions, as well as the error that arises by taking intoaccount different values of them, are examined. In fine textured soilsparameter c seems to be very small, so that one can easily suppose that the proposed equation reduces to the well-known Philips infiltration equation (Philip, 1957).

Evaluation of various computational schemes in calculating unsaturated hydraulic conductivity

Abstract The purpose of this study was to compare K - ϑ or K - H relationships (where K is the hydraulic conductivity, ϑ the volumetric water content and H the soil water pressure head) calculated by means of four computational models proposed, with experimental ones determined in situ in a layered soil profile. The experimental relationships have been reported by one of the authors before. The textural characteristics of the layered profile are shown. Furthermore, in order to evaluate the effect of differences between experimental and computed relationships on soil water profile development, calculated profiles for the process of horizontal infiltration obtained by using both experimental and calculated K ( ϑ ) relationships were compared. Also, cumulative infiltration versus time curves computed for the horizontal case by using both experimental and calculated K ( ϑ ) relationships were compared to the same end.

On the proper employment of Piche evaporimeters in estimating evapotranspiration

Daily meteorological measurements taken in Athens during the period 1951‐1984 are used to investigate the feasibility of calculating Penman-Monteith’s evapotranspiration estimates when wind measurements are not available. Daily sheltered Piche data (from a large meteorological screen) are shown to be satisfactorily related to the second term in Penman-Monteith’s equation on a daily basis. Furthermore, daily conventional Penman-Monteith evapotranspiration estimates are compared with the ones obtained from the adjusted Piche readings and the results show that daily, monthly or yearly evapotranspiration estimates, acquired over long periods, can be successfully estimated by using the substantial backlogs of Piche evaporimeter data. The analysis suggests monthly relationships to be justified for adjusting Piche data, although even one annual relationship seems to be able to estimate evapotranspiration in most of the months, or in a whole year period, satisfactorily.

A variable sorptivity infiltration equation

Horizontal and vertical one-dimensional infiltration are compared when they both occur in a homogeneous isotropic porous body initially at a uniform low water content θn under constant concentration (θ0) or constant pressure head (H0) conditions. From a consideration of the physics governing infiltration under such conditions, the conclusion is reached that the magnitude of the pressure head gradient atx=0, wherex=0 denotes the infiltration surface in the horizontal case, must be larger than the magnitude of the pressure head gradient atz=0, wherez=0 denotes the infiltration surface in the vertical case, for all finitet>0, so that for the hydraulic head gradient atz=0 to be greater than (1/2K0)Sxt−1/2 but smaller than [(1/2K0)Sxt−1/2+1],K0 being the hydraulic conductivity at θ0 andSx the sorptivity during horizontal infiltration. On these grounds, it is further argued that if the sorptivitySz is introduced for the case of vertical infiltration, then it must be equal toSx fort=0 only and that it must decrease with time. Results obtained by solving soil-water flow equations for the infiltration conditions defined above, and from experiment, support the above conclusions. An equation for the relationship between cumulative infiltration and time during vertical infiltration is developed after assuming thatSz decreases with time in an exponential manner. Cumulative infiltration versus time relationships given by this equation are compared with those obtained from the numerical solution of the soil-water flow equation and from experiment.

Effects of soil water potential on the nitrate content and the yield of lettuce

Abstract In order to investigate the effect of soil water tension (SWT) on the nitrate content and yield of lettuce, an experimental field was established on a clay loam soil in Southern Greece. Scheduling irrigation by tensiometers and/or porous blocks, water was applied in each of the four treatments when the SWT reached ‐30, ‐60, or ‐100 kPa, respectively. The fourth treatment was irrigated by using local plant and soil criteria. Lettuce fresh yield was increased as SWT was increased with maximum fresh wt at the SWT of‐30 kPa. The highest water‐use efficiency was obtained at the SWT of ‐100 kPa. The nitrate content in the external leaves was about 3.5 to 4 times higher than the content in the heart leaves of lettuce. Furthermore, the nitrate content from ‐30 to ‐100 kPa treatment was decreased 26% for the external and 23% for the heart leaves in dry matter and 22% and 19% in fresh matter, respectively. The total nitrogen (N) of the external leaves was decreased with decreasing soil water potential and ...