A framework based on Melisenda’s aridity index and on Budyko’s curve to assess the crop proneness to the hydrological sustainability

Many agroecosystems are experiencing an increase of agricultural water demand which risks depleting the natural reservoirs, viz lakes and aquifers. The increasing temperature reduces glacier extent and snow accumulation, thus reducing the dry—season water availability, and challenging the agricultural systems and the food security, particularly in arid and semiarid regions.

Aiming at contributing to defining effective strategies that are able to provide robust and parametric parsimonious estimates of the irrigation water demand of the agroecosystems at the planning level, we propose a framework based on the joint use of Melisenda’s aridity index, Benfratello’s water balance and Budyko’s curve to define the crop proneness to the hydrological sustainability.

The strategy is based on Benfratello’s (1961) explicit and conservative method to assess the soil water balance and the irrigation deficit in semiarid Mediterranean climates. The method is parametrized by means of an aridity index (Melisenda, 1964) to assess the soil proneness to water surplus formation, and the results are compared with the natural ecosystem deficit as provided by Budyko’s (1974) curve. Coupling these climatic water balances with a crop based estimate of the maximum required evapotranspiration, as given by the FAO procedure, it is possible to assess the expected crop irrigation deficit.

Our strategy is two—step. The first step is mapping Melisenda’s index, to identify the climatically—wet areas and the potentially climatically—dry areas. In potentially dry areas field capacity may not be refilled during the dry season, if it is greater than a critical value. It is worth noting that the greater is the field capacity, the smaller is the surplus water, and the greater is the crop water availability during the dry season. These maps may be produced both for the actual cultivations, and for some reference crops, viz millet, barley, rice and wheat, which are important for food security, to depict the local hydrological attitude to them.

The second step is the calculation of the monthly and annual irrigation deficit by means of Benfratello’s water balance. The irrigation deficit does not depend only on the annual precipitation and on the annual crop water demand, but also on their annual regime. Benfratello’s irrigation deficit is then compared with the ecosystemic water deficit, provided by Budyko’s curve. The closer is the crop behaviour to Budyko’s curve, the closer is its water demand to the ecosystemic one, considered as a reference natural water demand.

In order to test the sensitivity of the procedure at characterising the water balance also in presence of small climatic differences, we applied it with promising results to two important and comparable Mediterranean agricultural districts, the Bonifica della Capitanata (Southern Italy, 4,410 km², mainly cultivated with herbaceous crops, olives, fruit and grapevine trees) and the Mygdonia water basin (Northern Greece, 2,100 km², 1,030 of which are cultivated mainly with cereals). The Köppen—Geiger climate type is mainly Cfa for both areas. De Martonne aridity index depicts a semi—dry to Mediterranean condition for the Capitanata and a mainly Mediterranean condition for the Mygdonia.