A GIS based application of Benfratello's method to estimate the irrigation deficit in a semiarid climate

Benfratello's Contribution to the study of the water balance of an agricultural soil (Contributo allo studio del bilancio idrologico del terreno agrario) was firstly published sixty years ago, in 1961. The paper provides a practical conceptual and lumped method to determine the irrigation deficit in agricultural disctricts. Since then, it has been used in many areas in Southern Italy.

According to the method, percolation and capillary rise from the groundwater table are absent, and surface runoff happens only when the soil capability of storing water is exceeded. The method is therefore suitable for semiarid (and eventually arid) climates. Dry and wet seasons are defined on a climatic basis, as the season during which potential evapotranspiration is greater than precipitations (and the stored soil--water decreases), and that during which precipitations exceed potential evapotranspiration (and the stored soil--water inccreases).

With these hypotheses, Benfratello proposed to adopt a power--relationship (with power m greater or equal to 0) to assess the ratio between the actual and the potential soil--water loss, as a function of the ratio between the stored soil--water and the maximum available storage. The solution provides, in a simple closed form, the actual soil--water loss as a function of the potential loss (which is a climatic characteristic), and generalizes previous approaches, viz Thorthwaite (1948) and Thornthwaite and Mather (1955) ones, for which m = 0 and m = 1, respectively.

In this contribution we present a GIS based application of Benfratello's method to assess the soil water balance and the irrigation deficit of the semiarid Capitanata plane (4550 km², Southern Italy), one of the most important agricultural districts in Italy. A comparison between the method and previous results for the same region will be provided as well. Due to its simplicity and to the small number of needed parameters, Benfratello's method might be regarded to as an effective tool to assess the effects of climatic, landuse and anthropogenic change scenarios on the soil water balance and on the irrigation deficit.