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 model's 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.     

Simulation of Soil Moisture Profiles Using K(h) from Coupling Experimental Retention Curves and One-Step Outflow Data

In the present work, the effect on the soil moisture profiles development and the cumulative infiltration when three different equations for calculating the diffusivity versus volumetric water content, D(θ) function, from one-step outflow experimental data are used, is investigated. These D(θ) functions are coupled with moisture retention curves θ(h) (h being the soil matric potential) obtained independently for the determination of the hydraulic conductivity function K(θ) or K(h). The so obtained hydraulic conductivity function, together with θ(h), were employed in solving Richards equation numerically under constant flux conditions in one dimensional vertical infiltration process. Two different porous materials were used for this investigation. It is shown that the three different equations used for the prediction of K(h) have no significant effect on the shape of the moisture profiles for the sand mixture and that a Green–Ampt advancement of the wetting front is observed. For the case of sandy clay soil there are some noticeable differences in the moisture profiles and their shape is comparatively more effusive. Also, one could mention that for both porous media the time of incipient ponding (T) differentiates among the three equations used.     

Evapotranspiration measurements over a soybean field

Evapotranspiration data over a soybean field were obtained from a weighing lysimeter during 1988, when the month of July was marked by intense heatwave in Greece. Soil water contents were determined by neutron probe at regular intervals before and after irrigation. Meteorological measurements, including mean daily values of air temperature, relative humidity, wind speed, solar radiation and precipitation allowed the study of reference evapotranspiration and its comparison with class-A pan evaporimeter data during this period of extreme exposure conditions. Values of the Priestley-Taylor parameter, , were derived and a mean value of 1.58 was found. A generalized basal crop coefficient curve was determined from the lysimetric data and the time evolution of soil moisture profiles according to Wright's procedure. A rather high peak value, 1.4, for the soybean crop coefficient was obtained. This is mainly attributed to the very high crop evapotranspiration demands during the 1988 heatwave in Greece and the differences in the method of deriving the crop coefficient and computing the reference evapotranspiration.     

Tunnelling current versus voltage characteristics in metal-insulator-metal films

The tunnelling current j(V) and the logarithmic conductivity σ(V) ≡ (1/j)(dj/dV) are calculated for exactly solvable one-dimensional models of metal-insulator-metal structures. When the tunnelling probability is calculated by the MWKB method, the logarithmic conductivity exhibits a single peak at a value of the applied voltage equal to the barrier height. When the tunnelling probability is calculated exactly this peak is shifted to a higher value of the applied voltage and additional peaks appear at even higher values of the applied voltage, depending on the thickness of the insulating film. This additional structure in σ(V) is due to “resonances” in the tunnelling probability which may or may not survive in three a dimensional calculation.     

Evapotranspiration and sensible heat flux estimation above grass: Comparison of methods and correlation of several attributes to routinely measured data

Hourly lysimetric and micrometeorological data taken over a grass surface at the Meteorological Research Unit, Cardington U.K. have been analysed. Measured quantities include sensible heat flux and friction velocity, obtained directly by the eddy correlation method, wind speeds and temperatures at various heights, screen dry and wet-bulb temperatures, net radiation, soil heat flux at several soil depths and lysimetric data taken hourly by a weighing type lysimeter. A temperature and wind difference allowed sensible heat and momentum fluxes determination by the profile method on an hourly basis. Estimation of latent and sensible heat flux by the resistance method is also obtained. Use is made of the energy balance at the surface and of the Monteith-Penman formula for estimation of the latent heat flux. Equations based on the Monin-Obukhov similarity theory are used to account for stability effects. Aerodynamic and surface resistances, necessary for the Penman-Monteith equation are also calculated from routinely measured meteorological data.The comparison of the various methods leads to the establishment of empirical relationships which correlate various quantities such as soil heat flux, resistances, evapotranspiration etc. to routinely measured meteorological data. The experiment was performed under near neutral or slightly unstable conditions. It appears that, under these environmental conditions the profile method for estimation of sensible heat flux and friction velocity is found to work excellently. Surface latent and sensible heat fluxes can also be described very well by the resistance method. The results also substantiate the accuracy of the Penman-Monteith equation for estimating latent heat flux and its insensitivity to adjustments for atmospheric stability.     

Solute Transport in Soils: An Algorithmic Approach

An algorithmic approach is proposed for the problem of solute transport in soils. The algorithm is based on the assumption of steady-state water flow conditions and it is applied to both processes of salt accumulation and salt removal (leaching). Soil-solute interactions are also included in the algorithm. A basic assumption of the model is that the rate of salt accumulation or removal follows a first-order kinetic equation. The involvement of a piston-flow mechanism, observed frequently in laboratory or field experiments, may be accounted for, by the present model, through sub-dividing the soil column and applying a convolution integral to the various soil sections. Breakthrough curves calculated by the present approach could be adjusted so as to bring them in close proximity with the experimental ones already presented by other researchers.     

Canopy Resistance and Actual Evapotranspiration over an Olive Orchard

Τhis study evaluates the hourly actual evapotranspiration (AΕT), predicted either by the two modified Penman-Monteith models (PM) which take into account the canopy resistance (r_c) from the Katerji-Perrier (KP) or Todorovic (TD) models, or the simplified PM model with zero r_c, as proposed by Priestley and Taylor (PT). The evaluation is based on comparisons with experimental measurements of AΕT applying the ‘Bowen ratio’ method. Hourly experimental data, of air temperature, humidity, wind speed and radiation balance measurements, taken at a 0.5 ha olive orchard in the rural area of Sparta (37° 04? N, 22°05? E), during the period from June 2010 up to July 2014, are used. The r_c estimated by KP model is parameterized by a semi-empirical approach which requires a simple calibration procedure, while r_c from TD model is parameterized using a theoretical approach. For estimating AET from minimum data (air temperature, humidity and radiation balance components) the PT model is also employed, since r_c is not required and the aerodynamic term of PM is taken into account in the empirical parameter of the model. The results show that PT and KP models are the most appropriate [Refined Index of Agreement (RIA) equal to 0.89 or 0.88, respectively] followed by the TD model (RIA?=?0.78). PT or KP models underestimate AET by 9.3% or 9.8%, respectively, while TD model overestimates AET by 15.0%, increased up to 25.8%, during warm period.     

Evaluation of a Dielectric Sensor for Measurement of Soil-Water Electrical Conductivity

The salinity status of soil can be obtained through the measurement of the pore-water electrical conductivity σp. In the present study the WET sensor’s capability in predicting σp through the simultaneous measurements of the soil bulk electrical conductivity (σb) and the soil dielectric constant (K) is investigated. The estimation of σp is based on a model by Hilhorst, which relates σp to the two other quantities K and σb and an extra fitting parameter K0 which is incorporated in the software of the device. The study involved experimental measurements in the laboratory using four different soils with a large range in soil texture and volumetric water content θ. In each soil type four different electrical conductivity aqueous solutions were used. The results exhibited a rather strong linear relationship between K and σb. The slope of this linear relationship appeared to depend on both the soil type and the electrical conductivity of the pore water. The value of K0 seems to be soil specific and increases when the salinity level increases. The σp prediction according to the linear model of Hilhorst deviates for almost all soils tested, except for the case of the sandy soil. Besides the linear model of Hilhorst, the Malicki and Walczak linear model was also tested for all soils under investigation and the model of Munoz-Carpena et al. was tested for sand. Malicki and Walczak model performance at predicting σp values was approximately the same as the Hilhorst model for sand and slightly better for sandy loam. In general, one could argue that the linear models could predict σp with some accuracy for the cases of coarse porous media.     

The Effect of Treated Municipal Wastewater and Fresh Water on Saturated Hydraulic Conductivity of a Clay-Loamy Soil

The effect of irrigating with treated municipal wastewater (TMW) and fresh water (FW) on saturated hydraulic conductivity (Ks) was investigated. Ks was determined in undisturbed soil samples, collected from the upper soil layer (0–15 cm) of an illitic clay-loamy soil, after two irrigation periods. Soil samples were analyzed for a number of factors potentially affecting Ks. Analysis of variance for each of these factors was performed between the two treatments in order to detect their differences. Statistically significant differences were found for pH, ECe, SAR, ESP and the concentrations of K⁺, Na⁺, Mg²⁺ which are elements related with TMW and FW quality-characteristics. Correlation between all factors and Ks showed that this was positive for soil ECe, CaCO₃ and negative for SAR, ESP, and bulk density. Although the influence of each separate factor was investigated and the differences between the two treatments were verified, their quantitative characteristics could not explain based on existing criteria, (the Ks reduction observed between the TMW-Ks and FW-Ks). The 2:1 type clay minerals which are more susceptible to SAR increase and EC decrease affected Ks values between the two treatments. Our findings are in line with some recently reported results and in this sense, more specific research is needed to clarify all aspects relevant to Ks and the use of TMW for irrigation purposes.