Water balance and irrigation

Water is a finite resource: there are some 1.4 billion km3 on earth and circulating through the hydrological cycle. Nearly all of this is salt water and most of the rest is frozen or under ground. Only one-hundredth of 1% of the world's water is readily available for human use. In many countries, the amount of water available to each person is falling, as populations rise. Of the three main ways in which people use water:

• Municipal (drinking water and sewage treatment);

• Agricultural (mostly irrigation);

farming accounts for the largest part, some 65% globally in 1990. Irrigation systems have existed for almost as long as settled agriculture and they are essential to feed the world. Although only 17% of the world's cropland is irrigated, it produces over 33% of our food, making it two and a half times as productive as rain-fed agriculture. Nevertheless, in some cases up to 60% of the water withdrawn for use in irrigation never reaches the crops (Fig. 26.2). In addition, waterlogging and saliniza-tion have sapped the productivity of nearly 50% of the world's irrigated lands. Other problems include the accumulation of pollutants and sediments in large dams and reservoirs, and the fact that irrigation systems provide an ideal habitat for the vectors of waterborne diseases, tte key to improve irrigation lies in recycling waste water, proper drainage and especially in more efficient use of water.

Water balance models play a central role in most of the agricultural system models describing soil-crop-atmosphere interactions. Water is not only the dominant factor affecting crop growth, it also determines soil processes like transport of chemicals, biological activity and matter transformations, surface runoff, groundwater recharge and pest and disease development on soil and crop surface.

Agricultural production growth and stabilization, under conditions of the extreme climatic events occurrence, such as severe droughts, can be ensured by using many methods, the most important one being the irrigation, provided attention is paid to environmental preservation and protection. Soil moisture information is of interest to a wide range of users and is crucial for making informed decisions on land and water resources management. Many existing water balance models can be used to evaluate the soil moisture dynamics and soil water deficits at the rooting depth of the different agricultural crops, in order to provide information necessary in taking decisions on irrigation planning and management.

Transmission to farm 15%

Fig.26.2. Irrigation losses in agriculture (www.fao.org)

In the last years, the importance of water balance models to assess different decisions regarding the irrigation planning and management was considerably enhanced. Many models exist for calculations on irrigation management. In particular, four models were given as an example in COST Action 718 "Meteorology application for agriculture" of European Union final report: AMBAV, CROPWAT, IRRFIB, and SWAP (Kroes et al. 2006).

AMBAV (Agrarmeteorologisches Modell zur Berechnung der aktuellen Ver-dunstung-agrometeorological model for calculating the actual évapotranspiration) is part of the complex agrometeorological model toolbox AMBER (Lôpmei-er 1994) developed by the Agrometeorological Research Braunschweig (German Weather Service, Deutscher Wetterdienst - DWD). tte model calculates the potential and real évapotranspiration and the soil water balance for different crop covers. It is used for producing irrigation recommendations which are disseminated by the DWD via fax service for different soil types using hourly data from the meteorological station network, including weather forecast up to 5 days, tte model is designed to be used by local meteorological advisory services. It considers 13 different crops: winter wheat, spring wheat, winter barley, rye, oats, maize, sugar beets, potatoes, oilseed rape, grassland, fruit trees, coniferous and deciduous forest.

tte CROPWAT was developed by FAO (Allen et al. 1998). Its main functions are to calculate reference évapotranspiration, crop water requirements, irrigation requirements, scheme water supply, develop irrigation schedules under various management conditions, evaluate rainfed production and drought effects and evaluate the efficiency of irrigation practices. CROPWAT is meant as a practical tool to help agrometeorologists and irrigation engineers to carry out standard calculations for évapotranspiration, crop water-use studies and more specifically, the design and management of irrigation schemes. It allows the development of recommendations for improved irrigation practices, the planning of irrigation schedules under varying water supply conditions, and the assessment of production under rainfed conditions or deficit irrigation.

IRRFIB model calculates reference daily water balance for different regions and represents agricultural decision support tool in the frame of agrometeorological information system. Its recent development enabled quick and accurate transfer of information to end users. An open code solution developed in Linux platform is based on the PostgreSQL database. In the SAgMIS meteorological, soil, crop and agrotechnical data are integrated. Daily meteorological data from regional stations in the frame of national network are automatically delivered into the system. Reference évapotranspiration is calculated by Penman-Monteith equation using air temperature, wind speed, air humidity and net radiation.

tte model SWAP (Soil-Water-Atmosphere-Plant) is the successor of the agro-hydrological model SWATR and some of its numerous derivatives. It has a long history with the first publications in the year 1978. tte latest version was published as SWAP3.0 by Van Dam (2000) and Kroes and Van Dam (2003). Top soils show the largest concentration of biological activity on earth. Water movement in the upper soil determines the rate of plant transpiration, soil evaporation, runoff and recharge to the groundwater. In this way, unsaturated soil water flow is a key factor in the hydrological cycle, transporting large amounts of solutes, ranging from nutrients to all kind of contaminations, tterefore the model SWAP aims at an accu rate description of unsaturated soil water movement to derive proper management conditions for vegetation growth, irrigation conditions and environmental protection in agricultural and natural systems.

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