Dear Eyad,

The development and management of soil and water resources watershed of arid require knowledge of the hydrology of arid systems. Unlike predictive relationships on the behavior of watersheds in most populated humid temperate zone, the quantitative relations in watersheds of arid areas have not been well defined. The studies highlight some striking features that have important implications for the development. Among the most notable include:

 - The extreme variability of sporadic precipitation, both in time and in space;

 - Even greater variability of short-term runoff in ephemeral drainage systems;

 - Strong line losses in rivers beds dry and often very permeable;

 - Low humidity, strong incident radiation and high temperatures that cause the risk of very rapid evapotranspiration;

 - Streams in steep and significant alluvial sediments that produce high yields;

 - A rare vegetation and strong winds;

 - Strong wind erosion rate due to thermal gradients and lack of vegetation cover;

 - Shallow soils badly leached, often with high mineral content, which results in low levels of infiltration and salt runoff;

 - A water yield, which decreases as the size of the watershed increases, due to heavy losses in line and the small size of the units or "cells" of producing runoff precipitation.

I have visited, but not worked in truly arid areas, so my views are slanted to what I have read, seen on trips, learned.  Lack of rain = poor plant cover = poor root density = maybe poor infiltration = high potential for  non-wettable soils; and in areas with short term, intense rain, high runoff, potential for flash flood, severe channel erosion or gullying, limited riparian areas, limited habitat for life, just a few species that are adapted to the harsh conditions.  Soil development is limited.  If there is grazing, food is so scarce that overgrazing may be a problem.  Streams are typically dry, loosing streams with little or no groundwater access.  Permanrnt water holes with good watEr are in high demand, and potentially easily polluted.  ET demand is vey high, so plants and animals may have mechanisms to retain water, respond quickly to rain for reproduction and may almost go dormant.  Poisonous species cannot afford to loose prey, so they may be extra poisonous.  Organic accumulation on soils is generally limited, favors barrens of exposed soils.  Erosion can actually be higher in some arid areas than areas with ample rain.  Water for human uses, would typically be limited.  Water, where it is found is the most important thing for life and development, so planning or allocating its use is important --whoever controls the water, likely controls everything else.  It takes well adapted individuals and species to survive and flourish in the harshness.  However, I once heard a story of using soil scallops (rounded pits) to plant trees, capture rain for each plant that eventually grew to forest with flowing streams, etc.  plant recovery with native species may help reverse some of the issues so the areas are better able to support life.  Water capturing and storage may help too.  Natural recovery is limited.  If recovery is possible, there will be times such as drier periods, when everyone must sacrifice to help benefit and maintain the recovery, converting desert or barrens to vegetated or forested lands.  Analyzing options and  potential for recovery, conservation and careful management are very important considerations.  For those who try to convert them, the chance of failure may be high and depend on a high degree of commitment.

Thank you very much Bachir and Williams for the most fruitful answers...

1) arid areas are characterized by very limited precipitations (less than 100 mm / year) and when this happens extremes intensities (flash floods) - how to exploit/utilize this water is challenging from the cost-benefit analysis, how to prevent damages resulting from floods.

2) the fresh subsurface hydrology is more intriguing, as you may have fossil freshwater coexisting with saline water bodies. Problems of sustainability of the utilization (for how long can you produce freshwater and at what abstraction rates), and the migration of saline water bodies towards the fresh water ones.

3) How to exploit saline water aquifers (less than 10,000 ppm chloride concentration) these are valuable sources of water (for direct irrigation and or mixing with fresh water and or treated effluent). Understanding the behavior of these systems is key in the development of their utilization

Dear Ayad, Have a look of the attached papers, you may find them interesting

Thank you Huda and Jacob ...

Hi Eyad, in addition to the issues raised by the other colleagues, I would also mention the effect of solid transport that often leads to reservoir siltation and reduction of the amount of water available for agriculture. The high precipitation intensity and the daily rainfall extreme values are, for instance, the main cause of soil erosion and land degradation in the Shaelian region. To cope with these issues, Soil and Water Conservation (SWC) measures have been regularly employed in the Sahelian area. If you are interested in this topic have a look to this paper: https://www.researchgate.net/publication/257766213_Cost-Effectiveness_of_Soil_and_Water_Conservation_Measures_on_the_Catchment_Sediment_Budget-The_Laaba_Watershed_Case_Study_Burkina_Faso

Article Cost-Effectiveness of Soil and Water Conservation Measures o...

maybe the runoff generation mechanism.

Hello Eyad, the main problems of  arid regions hydrology are: The general characteristics of such regions, the nature of the hydrological processes that govern runoff, and the differences in these processes.

Lack of observed data provides the major problem for runoff modelling in arid regions.