Several hydrologic applications require reliable estimates of monthly runoff in river basins to face the widespread
lack of data, both in time and in space. The main aim of this work is to propose a regional model for the estimation
of monthly natural runoff series at ungauged sites, analyzing its applicability, reliability and limitations.
A GIS (Geographic Information System) based model is here developed and applied to the entire region of Sicily
(Italy). The core of this tool is a regional model for the estimation of monthly natural runoff series, based on a
simple modelling structure, consisting of a regression based rainfall-runoff model with only four parameters. The
monthly runoff is obtained as a function of precipitation and mean temperature at the same month and runoff at
the previous month. For a given basin, the four model parameters are assessed by specific regional equations as a
function of some easily measurable geomorphic and climate basins’ descriptors.
The model is calibrated by a “two-step” procedure applied to a number of gauged basins over the region. The
first step is aimed at the identification of a set of parameters optimizing model performances at the level of single
basin. Such “optimal” parameters sets, derived for each calibration basin, are successively used inside a regional
regression analysis, performed at the second step, by which the regional equations for model parameters assessment
are defined and calibrated. All the gauged watersheds across the Sicily have been analyzed, selecting 53 basins for
model calibration and using other 6 basins exclusively for validation purposes. Model performances, quantitatively
evaluated considering different statistical indexes, demonstrate a relevant model ability in capturing the observed
hydrological response at both the monthly level and higher time scales (seasonal and annual).
One of the key features related to the proposed methodology is its easy transferability to other arid and semiarid
Mediterranean areas; thus, the application here shown may be considered as a benchmark for similar studies. The
calibrated model is implemented by a GIS software (i.e. Quantum GIS 2.10), automatizing data retrieving and
processing procedures and creating a prompt and reliable tool for filling/reconstructing precipitation, temperature
or streamflow time series at any gauged or ungauged Sicilian basin. The proposed GIS plug-in can, in fact, be
applied at any point of the hydrographical network of the region, assessing the precipitation, temperature and
natural streamflow series (at the monthly or higher time scales) for a desired time-window
