A modeling-GIS approach for assessing irrigation effects on soil salinisation under global warming conditions

https://doi.org/10.1016/S0378-3774(01)00090-7Get rights and content

Abstract

Soil salinisation is very often due to excessive irrigation. However, irrigation is absolutely essential for obtaining reliable crop yields, particularly under predicted global warming conditions. A simple methodology for assessing the salinisation risk for any water management situation and under predicted global warming conditions is presented. The methodology is illustrated by the assessment of irrigation effects on soil salinity at San Antonio del Sur Valley, in the southeast of Cuba. Irrigation from a new dam will support agriculture in the Valley, but at the same time soil salinity is expected to increase. Soil electrical conductivity at several depths and topographical altitudes were used to create raster layers in a Geographic Information System (GIS), thus, determining the border of the saline-affected zones by a GIS analysis. Water-table depth at the border of the saline zones was assumed to be 2 m. The physically based SWAP model was used to predict future water-table depths after irrigation begins and under global warming conditions. Future temperature and precipitation daily values were calculated from a linear increase/decrease of the daily values corresponding to a typical year, according to a global-change forecast for the zone. Soil hydraulic properties were estimated from pedotransfer function and published soil data. Simulated results predict a fast water-table raise of 1 m, due to the increase of irrigation water. Borders of the new saline zones under these conditions (i.e. the places where the water-table is at a 2 m depth) were calculated using a digital terrain model, assuming that the water-table rose 1 m over the whole valley. According to the simulation results, the original saline zones of the valley will be enlarged from 31.4 to 96.8 ha 15 years after the scheduled start of irrigation. The methodology could be used by farmers and decision-makers to select the most suitable water management solution considering both economical and environmental criteria.

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

References (28)

  • H.J. Caspary

    An ecohydrological framework for water yield changes of forested catchments due to forest decline and soil acidification

    Water Resourc. Res.

    (1990)
  • Centella, A., Gutiérrez, T., Limia, M., Rivero, R., 1997. Climatic change scenarios for impact assessment in Cuba....
  • Doorenbos, J., Pruitt, W., 1977. Guidelines for Predicting Crop Water Requirements. Irrigation and Drainage Paper 24,...
  • A. Eatherall

    Modelling climate impacts on ecosystems using linked models and a GIS

    Climatic change

    (1997)
  • Cited by (0)

    View full text