**2. Hydrologic aspects of arid and semiarid zones**

Several basic conditions can be distinguished between arid regions and other zones from a hydrological point of view:


### **2.1 Low soil water retention, poor fertility and formation of desert crust**

Vast areas of bare soils, low annual precipitation, and few high intensity rainfall events with high kinetic energy characterize arid zones. The parent material and the degree of weathering determines the level and availability of nutrients, and the type of clay and sandy minerals; these in turn largely determine cation exchange, base saturation, the adsorption, and sometimes fixation of Phosphorus, texture, permeability, moisture retention and the stability of soil aggregates. As a result of its chemical and physical properties, a soil may harden on drying, form a surface crust that restricts the entry of water and air and inhibits seedling emergence, develop a subsurface layer or pan that restricts root growth, be susceptible to erosion, become acid or alkaline on drying, or have a subsurface horizon that hardens irreversibly on exposure. Positive correlation between soil moisture in every layer and precipitation, and negative correlation between soil moisture and air temperature are common patterns (Zhuguo *et al.,* 2000). Such relationships are gaining more relevance in global change-related studies. In this sense, Seneviratne et al. (2006) reported that the impact of atmospheric circulation modifications might be indirect, by imposing changes in the seasonal cycle of soil moisture, which in turn can lead to modied soil-moisturetemperature coupling characteristics, although years ago, Calvet et al. (1998) pointed out that further studies are needed to investigate the soil water content retrieval by simple surface schemes using estimations of the surface soil moisture and temperature.

Soil constraints to plant growth may be summarized as:


l. Physical loss of soil,

68 Current Issues of Water Management

Several basic conditions can be distinguished between arid regions and other zones from a

1. A few, often intensive, rain events with a low amount of overall precipitation, which causes most of the small rivers or seasonal streams to be active only a few months or

3. Commonly, the aquifer is characterized by a thick aeration (vadose) zone, the zone between the water table and the land surface, resulting in a deep water table, and 4. Exacerbation of salinity-related problems, in both, groundwater and soils. When these factors are misunderstood, the lack of development and mismanagement of water cause

Vast areas of bare soils, low annual precipitation, and few high intensity rainfall events with high kinetic energy characterize arid zones. The parent material and the degree of weathering determines the level and availability of nutrients, and the type of clay and sandy minerals; these in turn largely determine cation exchange, base saturation, the adsorption, and sometimes fixation of Phosphorus, texture, permeability, moisture retention and the stability of soil aggregates. As a result of its chemical and physical properties, a soil may harden on drying, form a surface crust that restricts the entry of water and air and inhibits seedling emergence, develop a subsurface layer or pan that restricts root growth, be susceptible to erosion, become acid or alkaline on drying, or have a subsurface horizon that hardens irreversibly on exposure. Positive correlation between soil moisture in every layer and precipitation, and negative correlation between soil moisture and air temperature are common patterns (Zhuguo *et al.,* 2000). Such relationships are gaining more relevance in global change-related studies. In this sense, Seneviratne et al. (2006) reported that the impact of atmospheric circulation modifications might be indirect, by imposing changes in the seasonal cycle of soil moisture, which in turn can lead to modied soil-moisturetemperature coupling characteristics, although years ago, Calvet et al. (1998) pointed out that further studies are needed to investigate the soil water content retrieval by simple

**2. Hydrologic aspects of arid and semiarid zones** 

weeks every year, and sometimes only once every few years.

environmental degradation and desertification (Sharma, 1998).

**2.1 Low soil water retention, poor fertility and formation of desert crust** 

surface schemes using estimations of the surface soil moisture and temperature.

e. Low water retention in some cases, but under specific conditions, water logging

Soil constraints to plant growth may be summarized as:

a. Inadequate moisture

d. Low base saturation

h. Salinity

i. Impermeability j. Shallowness

f. Poor fixation of phosphorus

b. Deficiencies/imbalances in nutrients c. Low cation exchange capacity

impeded drainage and poor aeration

g. Low pH (acidity) with high pH (alkalinity)

k. Textural problems (crusting, hardening, stoniness)

2. Loss of soil fertility and development of "desert crust" on the land surface.

hydrological point of view:


In arid soils, dryness and salinity, and their interaction under mismanaged irrigation, are the main causes of soil degradation. Effluent irrigation results in increased soil sodicity, because of the medium-to-high salinity and sodium concentration (Balks et al., 1998).

Bare soils exposed to rainfall are subjected to physical and chemical processes that change the hydraulic properties of the soil near the surface (Arie and Resnick, 1996). When the soil is dry, a hard layer is formed in the soil surface that is often called "desert crust", commonly enriched with calcite or silica. Desert crust decreases the infiltration rate of soils, thereby increasing runoff and soil erosion, reducing the availability of water through the root zone, and impeding seedling and plant growth (Figure 1.A). Other kind of crusts are formed in agricultural plots, irrigated with saline waters (Figure 1.B). Understanding the formation and properties of such crusts, as well as developing engineering methods to break it, are essential to control the runoff-infiltration (groundwater recharge) ratio and to maintain successful and sustainable agricultural activities. When desert crusts are a result of microbial activity, these kinds of crusts could help to protect the soil surface (Campbell et al., 2009). Besides, arid soils typically possess within a predominant sandy soil, a very low organic matter content with a consequent low fertility. In this sense, crop productivity under dryland conditions is largely limited by soil water availability. Soil organic matter (SOM) contents have been found to be a reliable index of crop productivity in semiarid regions because it positively affects soil water-holding capacity (Diaz-Zorita, et al., 1999).

Fig. 1. Soil salinity symptoms and consequences. A: Photograph of soil fines and saline crusts; Wyoming, USA (http://www.powderriverbasin.org/assets/). B: Degraded unfertile soil because of salt accumulation in Chametla Baja California Sur, México

### **2.2 Vegetation and livestock control**

Vegetation may be the most important control on water movement in arid soils. Because vegetation in arid regions is opportunistic, when the water application rate is increased, plant growth increases as it uses up the excess water. The opportunistic nature of desert vegetation is shown by a significantly higher concentration of vegetation in areas of increased water flow, such as in ephemeral streams and in fissured sediments or rock-beds.

Assessing Environmental and Social Dimensions of

the countries that use the river water downstream.

**2.5 Desalinization strategies** 

costs in the near future.

**3. Methods** 

Where:

Iwsc water scarcity index [-]

S desalinated water in Mm3

Q the annual available water in Mm3

W annual freshwater abstractions in Mm3, (M: millions)

Water Issues Through Sustainability Indicators in Arid and Semiarid Zones 71

source for water that is essential for agriculture, industrial use, and drinking water. For example, the rights to use the water of large rivers in Africa (e.g., the Nile) and in the Middle East (e.g., the Euphrates and Jordan) remain one of the major issues that govern the relations and conflicts between the countries upstream, where most of the river water discharges, and

The process to separate salts from saline waters or desalinization of either deep saline groundwater or seawater is a feasible alternative source for water in arid regions. However, the cost of desalinization remains higher than most other alternatives. A complex infrastructure is required, and the need for a close source of saline water makes this alternative impractical in many arid environments. The world's largest desalinization projects are in the Arabian Gulf (Saudi Arabia, United Arab Emirates, Kuwait), United States, and Japan, all which are wealthy countries with long seashores; lately, in Northwest Mexico some desalinating plants are being installed, with uncertainty about operational

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:

Iwsc = (W – S)/Q (1)

Q = R + α S Dup (2)

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 desertification, which turn to be economically irreversible.
