The effect of spatially distributed meteorological parameters on irrigation water demand assessment

https://doi.org/10.1016/S0309-1708(02)00100-8Get rights and content

Abstract

Geographic information systems technology was used to utilize meteorological data and crop distributions in order to estimate irrigation requirements. The irrigation requirements were estimated as the difference between crop evapotranspiration and effective rainfall. Crop evapotranspiration was evaluated as the product of reference evapotranspiration and the crop coefficient, while the reference evapotranspiration was calculated via the FAO Penman–Monteith method. Monthly effective rainfall was estimated from total monthly rainfall according to the method developed by the USDA Soil Conservation Service. For the purpose of this paper, the island of Crete in Greece was used as a case study. Based on individual meteorological stations as representative stations for the whole island the irrigation water demand was estimated at 362±135 Mm3 for the year 1991. The large variation is caused by the location and number of representative meteorological stations. A combination of up to 10 meteorological stations, chosen for geographical reasons as representatives of different regions of the island, resulted in a significant reduction of this variation. The irrigation water demand for the year 1991 was estimated, based on 10 stations, at 359 Mm3. The station of Lefkogia can be considered the representative station for the island. An increase in irrigation water requirements is projected to be 8–10 Mm3/year for Crete. These analyses should assist in the strategic planning of water use for irrigation, particularly in cases where the number of stations is inadequate, by identifying representative stations for different regions and projecting further irrigation water demand. The accurate estimation of irrigation water demand is also essential for developing a rational policy for sustainable water resources.

Introduction

Evapotranspiration, which is the sum of the volume of water used by vegetation (transpired), evaporated from the soil and the intercepted precipitation on vegetation [1], plays an important role in our environment at global, regional and local scales. Water entering the evaporation phase of the hydrological cycle becomes unavailable and cannot be recovered for further use [2]. In many areas, where water resources are scarce, the calculation of this loss becomes imperative in the planning and management of irrigation practices. Temperate areas, which have rainy seasons, are mainly concerned with shortages of water for multiple uses during dry seasons. Despite this significance, evapotranspiration is one of the least understood components of the hydrologic cycle [2], [3].

Evaporation and transpiration occur simultaneously and there is no easy way to distinguish between the two processes [4]. Transpiration consists of the vaporization of liquid water contained in plant tissues and the removal of that vapor to the atmosphere. Evaporation occurs from the topsoil if water is available. When the crop is small, water is predominantly lost by soil evaporation; once the crop is well developed and completely covers the soil, however, transpiration becomes the main process of water loss. Smith et al. [5] defined reference evapotranspiration as “the rate of evapotranspiration from a hypothetical reference crop with an assumed crop height (12 cm), a fixed crop surface resistance (70 s/m) and albedo (0.23), closely resembling the evapotranspiration from an extensive surface of green grass cover of uniform height, actively growing, completely shading the ground with adequate water”.

Brutsaert [2] and ASCE [6] have proposed numerous methods for modelling evapotranspiration. In general, the combination of energy balance and aerodynamic equations “provide the most accurate results as a result of their foundation in physics and basis on rational relationships” [6].

Geographic information systems (GIS) technology, represented in this case by the commercial package ArcView GIS 3.2, was used as it provided the tools for spatial data management, analysis, display, and interface functions. It helped greatly in conveying the message of this paper.

The purpose of this paper is to: (a) estimate the irrigation water demand by using GIS and incorporating the spatial and temporal variability of the relevant irrigation water demand parameters; (b) identify a representative station for Crete’s average irrigation demand; and (c) identify representative stations for different regions of Crete to be used as a guide for the strategic planning of water use for irrigation purposes.

Section snippets

Methodology

Reference evapotranspiration (ET0) is either directly measured, using a lysimeter, or estimated, using climatological or pan evaporation data. For the purpose of this paper, the FAO Penman–Montieth approach for calculating reference evapotranspiration (ET0) is used. The Penman–Monteith approach is a combination of both aerodynamic and radiation terms, as reported by Allen et al. [2] in paper No. 56 of the Food and Agriculture Organization of the United Nations (FAO). The general equations are:ET

Case study

Greece is divided for administration and management reasons into 14 hydrologic compartments (as shown in Fig. 1). Compartment 13 is the island of Crete, which is located at the most southern point of Greece. Approximately 7% of the total irrigated land in the country of Greece is located on the island of Crete. In spite of adequate precipitation, the increased demand for water for agricultural use in Crete, which consists of approximately 80% of the water use, cannot be always met [11]. This is

Analysis and results

Among the 14 hydrologic compartments of Greece, and based on meteorological parameters obtained for the stations of Iraklion and Rethymno the island of Crete was ranked as the third highest compartment with respect to the average consumption of irrigation water per unit area (as shown in Table 2). This demand for water can be accounted for by the large areas of trees and grapes, which require relatively higher amounts of irrigation water. According to the formula (Irrigation=A×Eto×Kc−A×Peff),

Conclusion

It is evident that the more the stations the better; however, due to the high cost normally associated with operating those meteorological stations, assigning representative stations for smaller regions/catchments is more appealing, especially in developing countries, where finances can be a problem. The representative station is generally a station that is located at a moderate altitude and operating under average meteorological conditions. Information from those representative stations can

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