A modeling-GIS approach for assessing irrigation effects on soil salinisation under global warming conditions
Introduction
The greenhouse effect will produce a general warming in the coming years (IPCC, 1996). Even though the total temperature increase is not expected to be high in the tropics, this effect will significantly increase the evaporative demand of crops (Scholes and Van Breemen, 1997). Therefore, irrigation water requirements will increase resulting in higher water-tables and increased soil salinity if the leaching fraction remains unchanged. Excessive irrigation has been shown to be the main cause of soil salinisation in more than one million hectares in the valleys of Cuba due to the raising of saline water-tables in lower lands (Ortega et al., 1982). Hence, an increase in irrigation water could increase the soil saline area in these Cuban valleys.
Although the spatial variability of soil salinity is generally high, the average levels often reflect topography. Lower lands are generally more saline due to shallow saline water-tables generated in part because of downward lateral flows from the irrigated higher surrounding lands. The relationship between salinity and topography has been used for sampling purposes (Utset et al., 1998), considering the slope as the maximum direction of electrical conductivity (EC) variability. Also, the relationship has been used for salinity risk studies with the aid of a Geographic Information System (GIS) (Bui et al., 1996), since the risk decreases with altitude. The minimum depth at which the water-table should be located, in order to avoid soil salinisation due to capillary action, is usually known from drainage and other related studies. Consequently, the areas of salinity risk could be estimated as those where the water-table depth is shallower than this minimum depth.
Water-table depths could be measured directly, monitoring the salinity risk in any zone. This kind of measurement has practical difficulties and, hence, in most agricultural areas in tropical countries there is no information available on water-table depths. Nevertheless, the water-table depth under any water management or climate conditions can be estimated through hydrological models. These models are able to estimate water-table depths, drainage flux, and many other hydrological variables by simulating soil water movement (Van Genuchten, 1994, Leenhardt et al., 1995). Models have been used to predict agricultural yields under global warming conditions (Wilks, 1988, Nonhebel, 1993, Semenov and Porter, 1995, Rosenzweig and Tubiello, 1996) and for calculating future hydrological variables in high risk catchment areas (Gleick, 1987, Caspary, 1990, Valdes et al., 1994, Rao and Al-Wagdany, 1995). However, until now, these models have not been used to predict water-table depths and salinity risks under global warming conditions. This paper shows a simple methodology for assessing future soil salinisation due to the increase of irrigation water under global warming conditions by using hydrological models in a GIS environment.
Section snippets
Materials and methods
The study was conducted at San Antonio del Sur Valley. It is located at 20°01′N and 75°16′W, in the southeast semi-arid zone of the island of Cuba. Annual precipitation records lower than 600 mm characterize the climate of the Valley. Precipitation is seasonally distributed. More than 80% of total annual rainfall is recorded between May and October. Mean daily temperature is 26.0°C. Average air humidity is 75%. Mean evaporation, as measured in a class A pan evaporimeter, is 2345 mm.
Soils of San
Results
A GIS analysis was performed in order to determine the water-table affected saline zones. These zones are those where EC is higher in the soil surface and decreases with depth. In those areas where EC increases with depth, the water-table is deep enough and there is, as yet, no salinisation risk. Therefore, a comparison among EC raster layers in the GIS was made, obtaining those pixels where the condition EC0–20>EC20–40>EC40–60>EC60–100 was met. Those zones are depicted in Fig. 3. As expected,
Conclusions
Farmers and decision-makers can rely on a simple methodology for assessing the salinity risk in agricultural lands where irrigation will be introduced/increased under global warming conditions. The methodology comprises the following general steps:
- 1.
Calculating the saline areas and their borders by using DTM and soil EC measurements at several depths.
- 2.
Estimating the time changes in the water-table depth through a hydrological model and considering a climate scenario.
- 3.
Predicting the new saline areas
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