**3. The AnnAGNPS model**

AnnAGNPS is a distributed parameter, physically based, continuous simulation, daily time step model, developed initially in 1998 through a partnering project between the USDA Ag‐ ricultural Research Service (ARS) and the Natural Resources Conservation Service (NRCS). The model simulates runoff, sediment, nutrients and pesticides leaving the land surface and shallow subsurface and transported through the channel system to the watershed outlet, with output available on an event, monthly and annual scale. Required inputs for model im‐ plementation include climate data, watershed physical information, as well as crop and oth‐ er land uses as well as irrigation management data.

Because of the continuous nature of AnnAGNPS, climate information, which includes daily precipitation, maximum and minimum temperatures, dew point temperatures, sky cover and wind speed, is necessary to take into account temporal weather variations. The spatial variability of soils, land use, topography and climatic conditions can be accounted for by di‐ viding the watershed into user-specified homogeneous drainage areas. The basic compo‐ nents of the model include hydrology, sedimentation and chemical transport.

The SCS curve number technique (USDA-SCS, 1972) is used within the AnnAGNPS hydro‐ logic submodel to determine the surface runoff on the basis of a continuous soil moisture balance. AnnAGNPS only requires initial values of curve number (CN) for antecedent mois‐ ture condition AMC-II, because the model updates the hydrologic soil conditions on the ba‐ sis of the daily soil moisture balance and according to the crop cycle.

**4.1. Cannata watershed**

*4.1.1. Geomorphological information*

The watershed covers about 1.3 km2

**Figure 1.** View of the Cannata watershed in proximity of its outlet.

ment were reported previously (Licciardello and Zimbone, 2002).

0.2 to 17.6 mm h-1.

The Cannata watershed, located in eastern Sicily, southern Italy (outlet coordinates 37 53'N, 14 46'E), is a mountainous tributary, ephemeral in flow, of the Flascio River (Figure 1).

Prediction of Surface Runoff and Soil Erosion at Watershed Scale: Analysis of the AnnAGNPS Model

an average land slope of 21%. The longest channel pathway is about 2.4 km, with an average

In a survey conducted at the start of experimental campaign, five different soil textures (clay, loam, loam-clay, loam-sand and loam-sand-clay) were recognized on 57 topsoil samples; clay-loam (USDA classification) resulted as the dominant texture. The soil satu‐ rated hydraulic conductivity, measured by a Guelph permeameter, resulted in the range

Continuous monitoring of land use has highlighted the prevalence of pasture areas (ranging between 87% and 92% of the watershed area) with different vegetation complexes (up to 15 species) and ground covers. Four soil cover situations can be distinguished: a high-density herbaceous vegetation (eventually subjected to tillage operations), a medium-density herba‐ ceous vegetation, sparse shrubs and cultivated winter wheat with a wheat-fallow rotation. More detailed information about the watershed characteristics and the monitoring equip‐

slope of about 12% (Figure 2). The Kirpich concentration time is 0.29 h.

between 903 m and 1270 m above mean sea level with

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The peak flow is determined using the extended TR-55 method (Cronshey and Theurer, 1998). This method is a modification of the original NCRS-TR-55 technology (USDA-NRCS, 1986), which is considered as a robust empirical approach suitable for wide varie‐ ty of conditions including those where input data might be limited as in the experimental watershed (Polyakov et al., 2007).

The AnnAGNPS erosion component simulates storm events on a daily basis for sheet and rill erosion based on the RUSLE method (Revised Universal Soil Loss Equation, version 1.5, Renard et al., 1997). The HUSLE (Hydrogeomorphic Universal Soil Loss Equation, Theurer and Clarke, 1991) is used to simulate the total sediment volume delivered from the field to the channel after sediment deposition.

The sediment routing component simulates sheet and rill sediment deposition in five parti‐ cle size classes (clay, silt, sand and small and large aggregates) on the basis of density and fall velocity of the particles and then routes sediment separately through the channel net‐ work to the watershed outlet as a function of sediment transport capacity (calculated by the Bagnold equation; Bagnold, 1966). A key assumption is that the aggregates break up into their primary particles once they enter the stream channel.

For the chemical component of the model, dissolved and adsorbed sediment predictions are assessed for each cell by a mass balance approach. Algorithms for nutrient (nitrogen, phos‐ phorous and organic carbon) and pesticide dynamics are largely similar to the EPIC (Wil‐ liams et al., 1984) and GLEAMS (Leonard et al., 1987) models.

More details on the theoretical background of AnnAGNPS are reported by Bingner and Theurer (2005).
