In order to determine the water needs of the cultivations of the Messolonghion area, first the mean daily rate of the reference evapotranspiration was calculated employing the Penman-Monteith equation according to FAO-56 .
0,408A(R -G)+ g 900 u2(e, -ea) ET = K n J T + 273 2V 1 ^ (32.1)
where; ETo is the reference evapotranspiration (mm/day), Rn is the net radiation at the crop surface (MJ m-2 d-1), G is the soil heat flux density (MJ m-2 d-1) which, for daily intervals, may be ignored, T is the mean daily temperature at 2-m height (°C), u2 is the wind speed at 2-m height (m s-1), es is the saturation vapour pressure (kPa), ea is the actual vapour pressure (kPa), es - ea is the saturation vapour pressure deficit (kPa), - is the slope saturation vapour pressure curve at temperature T (kPa °C-1) and - is the psychrometric constant (kPa °C-1).
The daily reference evapotranspiration was calculated by considering meteorological data from Messolonghion station for the last 42 years (1967-2009) as provided by the National Meteorological Service (NMS).
The estimation of crop evapotranspiration, ETc, incorporates the single or the dual crop coefficients in the following equation:
where; Kc is the single crop coefficient, which averages crop transpiration and soil evaporation. The crop evapotranspiration, (ETc), for the crops of Messolonghion valley for the irrigation period of year 2009 was calculated.
The cropping land covers an area of approximately 3,820 ha as shown Table 32.1.
The mean plant coefficients for maize, citrus fruits, and olives in development stage (Kc) were obtained from Papazapheiriou  who determined them for the climatic conditions prevalent in Greece. For alfalfa and vegetables, for which the plant coefficients for the climatic conditions of Greece have yet to be determined, the crop coefficients by stage of growth Kc, were used as given by Allen et al. .
As it shown in Fig. 32.2, the greatest crop evapotranspiration was that of alfalfa, followed by those of citrus fruits, maize, olives, and vegetables.
Net water needs for the crops were determined according to the following equation:
where; Pe refers to the portion of rainfall, which may be used by the crops and it is known as "effective rainfall", G is the contribution of the groundwater and S is the w ^ m water stored in the topsoil layer of the soil at the beginning of the germinative period.
In this study, Gw was considered equal to zero, since the underground water table in the Messolonghion area plain is very low as a result of excessive pumping.
Table 32.1 Total cropped area in Messolonghion in 2009
Crop Area (ha)
□ 1. Maize □ 2. Alfalfa ■ 3. Vegetables □ 4. Citrus □ 5. Olive
Fig. 32.2 Crop évapotranspiration (ETc), in Messolonghion area (in mm/irrigation period)
The concession was also made that the soil moisture at sowing and at harvest time was the same, and the term Sm was, therefore, assumed to be zero. Beyond, however, the obvious need for water, which must be met through irrigation, is necessary for the leaching of salts, which are concentrated in the root-supporting layer of the soil.
Consequently, for the calculation of the net water requirements, the difference between the crop evapotranspiration of the cultivations minus the "effective rainfall" P was calculated.
The effective rainfall was calculated by the USDA  method, as follows:
where; Pe = average monthly effective precipitation (mm), Pt=monthly mean precipitation, ETc=average monthly crop evapotranspiration (mm), f(D) = soil water storage factor.
The soil water storage factor is defined by:
f(D) = (0.531747 + 0.295164 D - 0.057697D2 + 0.003804D3) (32.5)
where D=the usable soil water storage (mm).
Finally, the total water demand of the crops for the 2009 irrigation season was calculated on the basis of the crop statistics (data) for the same year.
The calculation of the total water demand for each crop for the entire season of irrigation was obtained by multiplying the net needs of water for irrigation of the various crops by the corresponding areas presented in Table 32.2.
Among all crops, vegetables were the most demanding crop for irrigation water, whereas olive was the last one. Considerable demands were also found for alfalfa, maize and citrus.
The pie chart of Fig. 32.3 gives the irrigation requirement by each crop as a percentage of the total water for the year 2009.
Table 32.2 Total demand for irrigation water by the crops of the valley of Messolonghion, 2009
Net demand for irrigation water, In of the crops (mm/period of irrigation)
316 382 469 280 225
Fig. 32.3 Percentage of irriga- Citrus Olives tion requirement for each crop -. 11%
Vegetables \ 34%
The total water requirements of these crops during the irrigation period of the year 2009 for the 3,820 ha of irrigated areas in Messolonghion are 12,697.033.33 m3, with an average of 3,323.8 m3/ha.
Given that the WWTP has a total of 1,689.950 m3 effluents the percent of reuse is estimated to be 13.3%.
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