**3. Methods**

70 Current Issues of Water Management

Where the water supply is limited, plant activity decreases until the water-supply rate increases. The importance of vegetation on a local scale has been shown in several field studies elsewhere, including soil cover protection, maintaining the soil aggregation and other effects. As a negatively associated activity, the extensive production and maintenance of livestock generate overgrazing, lose of plant cover, soil exposure, lose of biodiversity and

A number of studies of the vadose zone in arid environments have been conducted elsewhere primarily for water resources evaluation. In the last two decades of the twentieth century, however, emphasis shifted from water resources to waste disposal and the transport of salts and other contaminants. Arid areas are being proposed for low-level and high-level radioactive waste disposal. Most of the studies related to the vadose zone in arid settings were conducted in the western United States, in regions that are designated as waste facilities. Some of these sites include Hanford Washington, Sandia New Mexico, Ward Valley California, Eagle Flat Texas, Nevada test site and Yucca Mountain Nevada (Scanlon et al., 1997). The increasing interest in the desert environment for waste facilities, in general, and radioactive waste, in particular, raises the need to understand the importance of preferential flow in the subsurface. One could assume that a thick vadose zone combined with low precipitation promotes the safest possible environment for waste disposal. However, fast flow via fractures, cracks, and macropores had been suggested as a major mechanism leading to contaminant transport much faster than anticipated by models that predicted transport based on average soil properties. For hydrologic studies, dual-porosity models exist (i.e. HYDRUS) (Simunek, 2008), but they are difficult to parameterize for this kind of soils.

Salinization is a significant issue to consider in arid environments is the salinization of both soils and groundwater. The low precipitation combined with high evapotranspiration and often-slow flow rates through the subsurface, result in higher concentrations of salts. Human-induced salinization has a long history. A major source of salts accumulating in the upper vadose zone is irrigation water, which is essential for sustaining agriculture in arid lands. More than one-third of the developed agricultural lands in arid and semiarid regions reflect some degree of salt accumulation. High salinity in agricultural lands imposes stress on the growing crops that can lead to decreased yield and in some cases complete crop failure. This problem emphasizes the need for careful management of desert land and water

Despite the difficulties for plants, animals, and humans to live in desert regions, they are increasingly being utilized because of pressure from world population growth. This problem is expressed in the expansion of agricultural activities onto desert lands as well as by the formation and rapid growth of urban and industrial centers. These trends not only result in a growing demand for usable water, but also for the increased disposal of vast amounts of wastewater and solid wastes (e.g., radioactive wastes, hazardous wastes, and municipal solid wastes). In several cases, international conflicts have developed due to water rights in arid regions. Large rivers crossing desert regions are often the only potential

desertification, which turn to be economically irreversible.

**2.3 Vadose zone studies** 

**Salt accumulation** 

balance.

**2.4 Water management issues** 

Sustainability indicators were reviewed and applied to an arid region; a study case was analyzed by means of the application of selected indicators and indexes in an overexploited aquifer. Results were interpreted within the framework of sustainability of water resources. A water usage balance study was analyzed for the La Paz Watershed, Baja California Sur, in a semiarid zone of Northwest Mexico, in order to determine environmental and social dimensions of water issues through sustainability indicators. In this zone, conventional crops are a major user of irrigation water, because of its water-demanding nature, due to an average of the five to seven irrigations needed per year. Within the La Paz watershed, four micro basins were evaluated for water deficit: El Cajoncito, La Huerta, La Palma and El Novillo.

Three variables were assessed in order to estimate the index of water scarcity Iwsc (water availability indicator), a composed integrated index which takes into the regional hydrological account, the natural groundwater recharge, the extraction and the resulting balance. In order to understand the relationship 'availability-demand', the index of water scarcity (Iwsc), which combines information about water abstractions and water availability, is assessed at first. For this purpose, the regional water availability index (Irwa) is a measure of water available for socio-economic development and agricultural production. It is the accessible water diverted from the runoff cycle in a country, region or drainage basin, expressed as volume per person per year, m3/p/y. The indicator Iwsc is defined by:

$$\text{Iwsc} = (\text{W} - \text{S}) / \text{Q} \tag{1}$$

Where:

Iwsc water scarcity index [-]


$$\mathbf{Q} = \mathbf{R} + \alpha \mathbf{S} \,\mathrm{Dup} \tag{2}$$

Assessing Environmental and Social Dimensions of

city. Northwest México

Water Issues Through Sustainability Indicators in Arid and Semiarid Zones 73

Fig. 2. Geographic location of the Mexican State Baja California Sur and La Paz, the capital

For the application of sustainability indicators to the diagnosis of La Paz watershed, data on water uses, the natural groundwater recharge and the extraction were obtained from the National Water Commission (CONAGUA) reports and other previous studies (Cruz-Falcón, 2007; CONAGUA, 2008). Weather data (temperature, evaporation and precipitation) used in this study for La Paz B.C.S. (México) were obtained from División Hidrométrica de Baja California Sur, of the Comisión Nacional del Agua (CONAGUA, 2008), who collects this information from the La Paz Weather Station, located at 24°09'N and 110°20'W, 3 km south La Paz City. Our analysis indicate that agriculture is the major water consuming activity, in

The natural groundwater recharge was estimated by the method according to the groundwater lever fluctuation method, which is an indirect method of deducing the recharge from the fluctuation of the water table. The rise in the water table during the rainy season is used to estimate the recharge, provided that there is a distinct rainy season with

**Water use Baja California Baja California Sur Peninsula (total)**  Agriculture 1830227 6450 1836677 Domestic 74 1914 1988 Multiple (industries) 164 425 589 Livestock 809 2403 3212 Urban-public 146640 421 147061 Services (county) 28 129 157 Total 1977942 11742 1989684

Source: Official data base from REDPA - National Water Commission of Mexico (CONAGUA, 2008). Table 1. Synopsis of the water use in the Baja California Peninsula and related States,

both, Baja California Sur state and the whole Peninsula of Baja California (Table 1).

the remainder of the year being notoriously drier (Cruz Falcon, 2007).

according to the consumptive use; unit: Millions m3/yr

Where:

R the internal water resources in the country in Mm3

Dup the amount of external water resources in Mm3

α ratio of the external water resources that can be used.

The factor α is influenced by the quality of the transboundary water, by the consumption of water resources in the upstream region, and the accessibility of water.

Critical values of Iwsc identify various ranges for water scarcity and its parameters; the most common range for Q oscillates between 1000 and 1700 m3/p/y. A region is considered highly water stressed if Iwsc is higher than 0.4 (Alcamo et al., 2003), which is a reasonable although not definitive threshold value, because not all the renewable freshwater resources are used by human society. Data with shorter time scales will enable more detailed assessments considering the effects of seasonal variability in the hydrological cycles (Oki, 2006). These values are important because the World Bank and other aid organizations use them to prioritize and to direct aid to developing nations.

An indicator related to the 'efficiency of land cultivation' is the cultivation factor R.

The ratio of cultivated to non-cultivated land was defined by Ruthenberg (1976) as:

$$\mathbf{R} = (\mathbf{C} \times 100) / (\mathbf{C} + \mathbf{F}) \tag{3}$$

Where:


For the interpretation of R, Ruthenberg defined:

R < 30 as shifting cultivation;

R = 30 to 70 as semi-permanent cultivation;

R > 70 as permanent cultivation.
