**5. Conclusions**

76 Current Issues of Water Management

obtained from an aquifer, it is crucial to analyze the concept of sustainability of an aquifer: Maintaining a balance between recharge and extraction, or seeking that average annual extraction does not exceed the annual recharge. Although extraction from some groundwater resources has been above the long-term sustainable yield, its use must be managed to the sustainable yield through the implementation of water sharing plans for groundwater (Scanlon, 1997). Current droughts have increased the demand on groundwater resources, causing localized stresses in parts of groundwater systems and overexploitation, with a high risk of water scarcity. A wide variety of ecosystems depend on groundwater for their continued survival. Significant changes in groundwater quality and quantity have the

Fig. 4. Geographic and topographic configuration of four microbasins within La Paz BCS

Agriculture and livestock depend on water. Under natural conditions, water deficit is a common condition for agro-ecosystems and grazing-lands in arid zones. "Rainfall" is an important climatic parameter, but in arid and semiarid zones, its analysis scarcely explains the dryness intensity and the aridity pattern. At this stage in earth's history, it is believed that mankind can make 'productive efforts' to decrease or diminish the water depletion, such as artificial groundwater recharge, small dams, artificial infiltration ponds, desalination, increasing the water use efficiency, mitigating or remediating losses, promotion of native agro-forestries for water retention, ad others. In a sort or medium term, the hydrological

watershed, Northwest Mexico, with estimated water deficit, in millions (M) m3

potential to degrade ecosystems and affect human uses of water.

The study reported here has applied a series of spatially-resolved data sets depicting the biogeophysical and socioeconomic properties of the South-Baja Californian communities in Northwest Mexico. We, as others, have found that Northwest Mexico is a dry zone. Associated with this dryness is a highly dynamic water cycle, providing a large degree of variability in terms of climate, runoff and discharge. A systems perspective is essential to understanding sustainability. The system is envisioned in its broadest sense, from the individual farm, to the local ecosystem, and to communities affected by this farming system, both locally and globally. An emphasis on the system allows a larger and more thorough view of the consequences of farming practices on human communities and the environment. The application and interpretation of sustainability indicators motivates, for the La Paz watershed case, the design and instrumentation of strategies in order to improve the water use efficiency, and to alleviate the water deficit of the aquifer.

A significant fraction of agricultural land and human population is located in the study region with low runoff hydrography at the center of the La Paz valley, with disperse small localities and villages at the high sections of the watershed, with high runoff and high variability. Hence, agricultural water demand defines the aggregate water use for the watershed. These characteristics of human-water interactions, in turn, provide challenges to the water infrastructure of the watershed, with evidences that the region may be experiencing curtailed use of water, relative to its high demands. Biogeophysical data sets, emerging rapidly from the local science community, can make important contributions to emerging water resource assessments.

On the base of available evidences, we conclude that both, bio-geophysical as well as socioeconomic indicators will be necessary to map the patterns and intensities of water scarcity. Interdisciplinary study is thus an important component of future research.
