Mustapha Besbes

From infiltration to recharge: use of a parametric transfer function

Abstract The recharge of a water-table aquifer involves transfer of the infiltration (effective rainfall) through the unsaturated zone. In certain areas the depth at which the water table lies and the properties of the unsaturated zone can vary greatly and thus the determination, as a function of time, of the recharge actually arriving in the saturated aquifer can be rather difficult. However, we need to know this recharge in order to simulate the true behaviour of the aquifer in transient state in a regional digital model of the aquifer for the purpose of calibration or prediction. The resolution of the exact flow equations in the unsaturated zone is possible, but seems still to be far too complex for practical applications. In this paper we first propose a methodology for the identification of the spatial variability of the infiltration transfer function using common climatic and piezometric data. A parametrisation of these transfer functions using gamma distributions as basic functions is then suggested. Two practical examples make it possible to check the applicability of the procedure.

A comprehensive water balance of Tunisia: blue water, green water and virtual water

This paper envisions a comprehensive balance for water management that takes into account all kinds of water resources: withdrawal (blue) water, (green) water in rainfed agriculture and the net contribution of “virtual water” to the food import–export balance. In countries where water resources are limited, this leads to a better understanding of the relation between water and food production and gives a clearer picture of the potential for agricultural production. In the case of Tunisia, it indicates that food security will depend on the ability of the country to manage all the available water resources, in particular by improving the potential of rainfed agriculture.

Changing Water Resources and Food Supply in Arid Zones: Tunisia

The notion of water security in an arid country takes on another dimension when the comprehensive water balance concept is applied to water used by rain-fed agriculture and to the water equivalent of international food exchanges. In the case of Tunisia, this concept expands the prospects for improvements in national food security by optimizing the food balance and the corresponding virtual water flux. It also prompts reconsideration of criteria and indicators classically used to characterize water stress situations. The current situation shows that about 30% of the water used in Tunisia is imported as food (virtual water); that number is likely to reach 40–50% in 2025 due to climate change, diet change, demographic growth, and improved water management. Asia and North Africa will most likely not be self-sufficient in terms of food production and will need to import food from other continents (e.g., South America). Africa, however, could be self-sufficient if its existing water resources are developed. Bioenergy production is likely to be limited to a small fraction of the global energy needs. Major food shortages in cases of severe global droughts (e.g., during very strong El Nino events) may occur, however, with severe consequences in terms of food availability.

Virtual-water content of agricultural production and food trade balance of Tunisia

This article is devoted to the assessment of Tunisian agricultural production and food trade balance water-equivalent. A linear regression model relating annual rainfall to crop yields is developed to estimate the agricultural production water-equivalent. Its implementation is based on national data for crop and animal production, leading to food demand water-equivalent quantification. Results highlight the relationship between agricultural and water policies and provide a picture of food security in the country in relation to local agricultural production, and to virtual water fluxes related to foodstuffs trade balance.

National Water Security

ion of freshwater resources is at the basis of much of the social and economic development of humanity. But the real renewable freshwater resources of the Earth are represented by precipitations on the continents (Fig. 1.1). Estimated at about 110,000 km3/year on land, they perpetuate the renewable water resources of the Earth. These flows feed the continental hydrological cycle in its natural and human parts, the main term of which provides continental evaporation and evapotranspiration: through the soil water reserves, this term sustains life of continental ecosystems, and allows the development of rainfed agriculture. According to the soil moisture content and its texture, the rest of rainwater is subdivided into: (i) runoff to rivers and surface water systems, (ii) infiltration in the soil and at depth to aquifers. At the regional level, the continuous mapping of the soil water reserves is used to establish the link between meteorological and hydrological information with applications in the fields of: (i) agriculture, by detecting plant water needs for irrigation monitoring; (ii) urban hydrology and flood forecasting in case of extreme runoff conditions; (iii) hydrological and climate modeling, at regional and global scales. For all these applications, and in the same way that there are approved global networks of rainfall gauging stations, a global soil moisture database is taking place (Robock 2011). The soil water reserve makes the link between the amount of water precipitated on land, the available energy and the carbon cycle. In terms of food production and ecosystem functioning, it is the soil water resource which, by means of solar energy and generation of photosynthetic products, determines the Terrestrial Net Primary Fig. 1.1 Average annual precipitation, 1960–1990, mm/year; (based on data fromwww.worldclim. org, consulted on 2 June 2011) 8 1 The World Water Issues Table 1.3 Terrestrial soils occupation and net primary production (Adapted from Michigan University 2017) Ecosystem Extension 106 km2 TNPP dry matter 109 ton/year HTNPP dry matter 109 ton/year HTNPP (%) Forests 40 48.7 13.5 28 Prairies and Rangelands 35 52.1 11.6 22