Author: Victor Jetten

Introduction

The Zeuss Koutine area in Tunisia suffers from over exploitation of the aquifers, and extension of orchard cultivation at the expense of natural grazing lands. Severe long drought periods reduce soil water content to levels where olive plantation can suffer enormously. Traditional water harvesting techniques (Jessour and Tabias) are used for the improvement of water content of soil. Replenishment of groundwater aquifers are ensured through the recharge structures (gabion check dams and recharge wells). However, current cropping levels versus water availability may not be sustainable. The experiments are geared towards monitoring water levels, as the water harvesting techniques are well established. Moreover, ever since the ground water has been exploited by means of drilling a lot of pastoral land was converted into irrigated cropland or orchard. This has increased the pressure on the remaining land causing over grazing and associated soil erosion problem. An experiment was carried out to improve plant cover and biodiversity in the grazing areas aiming at minimizing land degradation.

Desertification indices
The two desertification risk indices investigated here are soil erosion, water stress and overgrazing. Soil erosion is not a problem on the fields where the technologies are tested but occurs elsewhere in the area. In the first area the calculations do not make much sense. The soil erosion risk is calculated as moderate (3.2) while the precipitation is very low (although the seasonality is high), there is terracing so the slope angle is < 2%, the soils are well drained soils and there is efficient runoff capture. On the other hand the water stress risk is indicated as no risk (0.99), but for this index the rainfall amount is not included, only the seasonality! The amount of rain does not seem to play a role with water stress in the system. Water harvesting techniques are not included, unless this decreases groundwater use in which case the water stress risk remains at a “no risk” level. In the south of the area heavy grazing takes place and the overgrazing risk is classified as low (2.11). When rangeland restingis implemented the set aside area has a large increase in cover and biomass. This decreases the overgrazing risk to 1.78 (still “low risk”).

Olive trees in a Tabias water harvesting system	Extensive grazing, unmitigated

Conclusions

The desertification risk assessment tool is not well equipped to deal with water stress in this very dry area. The factors leading to water stress are statistically chosen which creates some strange omissions (e.g. rainfall amount is not a factor in water stress). The grassland improvement is better simulated with an increase in cover as a result of rangeland resting.