**4. Description of the Experimental Watersheds**

The input data utilised for AnnAGNPS implementation in the Cannata watershed was col‐ lected during a proper monitoring campaign providing topographic, soil and land use data as well as 7-year hydrological observations.

For model verification in the Ganspoel watershed the input database was drawn from the works by Steegen et al., 2001 and Van Oost et al., 2005. Compared to the Cannata watershed, this experimental database reported less geomorphological information; moreover, the hy‐ drological observations were related only to a 2-year period: thus this study case represents a typical "data-poor environment" (Merritt et al., 2003).

#### **4.1. Cannata watershed**

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‐

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

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 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

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‐

More details on the theoretical background of AnnAGNPS are reported by Bingner and

The input data utilised for AnnAGNPS implementation in the Cannata watershed was col‐ lected during a proper monitoring campaign providing topographic, soil and land use data

For model verification in the Ganspoel watershed the input database was drawn from the works by Steegen et al., 2001 and Van Oost et al., 2005. Compared to the Cannata watershed, this experimental database reported less geomorphological information; moreover, the hy‐ drological observations were related only to a 2-year period: thus this study case represents

sis of the daily soil moisture balance and according to the crop cycle.

watershed (Polyakov et al., 2007).

6 Research on Soil Erosion Soil Erosion

the channel after sediment deposition.

Theurer (2005).

their primary particles once they enter the stream channel.

liams et al., 1984) and GLEAMS (Leonard et al., 1987) models.

**4. Description of the Experimental Watersheds**

a typical "data-poor environment" (Merritt et al., 2003).

as well as 7-year hydrological observations.

#### *4.1.1. Geomorphological information*

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).

The watershed covers about 1.3 km2 between 903 m and 1270 m above mean sea level with an average land slope of 21%. The longest channel pathway is about 2.4 km, with an average slope of about 12% (Figure 2). The Kirpich concentration time is 0.29 h.

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

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 0.2 to 17.6 mm h-1.

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

At event scale, rainfall depths over 6.8 mm gave runoff volumes higher than 1 mm; the max‐ imum runoff volume and discharge recorded in the observation period were 159.6 mm and 3.4 m3 s-1 (2.6 l s-1 km-2) respectively. Twenty-four erosive events were sampled with a sus‐ pended sediment concentration between 0.1 and 9.2 g l-1; the maximum event sediment yield (estimated on the basis of runoff volume and suspended sediment concentration in the flow)

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

http://dx.doi.org/10.5772/50427

9

The Ganspoel watershed (outlet coordinates 50 48'N, 4 35'E), located in central Belgium,

which can locally exceed 25%. A dense network of dry channels characterizes the area (Fig‐ ure 3). The topography of the area is formed in sandy deposits overlain by a loess layer that was deposited during the latest glacial period. Soils are therefore dominantly loess-derived luvisols, with their physical parameters related much more to land use than to soil texture

Top soils have a very high silt percentage (on the average 75%) and moderate clay and sand

The watershed land use is mainly agricultural. Forested (5%) and pasture (4%) zones cover the steep slopes as well as some of the thalweg areas. A built-up zone is located in northwestern part of the Ganspoel watershed and represents 9% of its area (Steegen et al., 2001).

The climate of central Belgium shows relatively cool summers and mild winters resulting in an average annual temperature of 11 C. Annual precipitation varies normally between 700

content (on the average 11% and 14% respectively) (Van Oost et al., 2005).

**Figure 3.** Location and aerial view of the Ganspoel watershed.

*4.2.2. The hydrological database*

between 60 m and 100 m a.s.l. with an average slope of about 10%, but

was 283 Mg (2168.4 kg ha-1).

*4.2.1. Geomorphological information*

**4.2. Ganspoel watershed**

covers 1.15 km2

The main

(Van Oost et al., 2005).

**Figure 2.** Location, contour map and hydrographic network of the Cannata watershed.

#### *4.1.2. The hydrological database*

In the monitoring period of 1996 to 2003 the hydrological observations were collected utilis‐ ing the following equipment (Figure 2): a meteorological station (A, located outside of the watershed) recording rainfall, air temperature, wind, solar radiation and pan evaporation; two pluviometric stations (B and C); and a hydrometrograph (D) connected to a runoff wa‐ ter automatic sampler (E) for the measurement of sediment concentration in the flow.

In the observation period yearly rainfall between 541 and 846 mm (mainly concentrated from September to March) was recorded at the station A, with a mean and standard devia‐ tion (SD) of 662 and 134 mm respectively. The corresponding yearly runoff was in the range 30.7 to 365.8 mm, with a mean of 105.3 mm and SD of 100 mm. The coefficient of yearly run‐ off, calculated as the ratio between total runoff and total rainfall as recorded by station A, varied between 5% and 41%, with a mean and SD of 15% and 75% respectively. Occasional high differences in recorded rainfall events between the three gauges were found; as expect‐ ed, rainfall spatial variability decreased on a monthly and yearly basis.

At event scale, rainfall depths over 6.8 mm gave runoff volumes higher than 1 mm; the max‐ imum runoff volume and discharge recorded in the observation period were 159.6 mm and 3.4 m3 s-1 (2.6 l s-1 km-2) respectively. Twenty-four erosive events were sampled with a sus‐ pended sediment concentration between 0.1 and 9.2 g l-1; the maximum event sediment yield (estimated on the basis of runoff volume and suspended sediment concentration in the flow) was 283 Mg (2168.4 kg ha-1).

#### **4.2. Ganspoel watershed**

### *4.2.1. Geomorphological information*

The Ganspoel watershed (outlet coordinates 50 48'N, 4 35'E), located in central Belgium, covers 1.15 km2 between 60 m and 100 m a.s.l. with an average slope of about 10%, but which can locally exceed 25%. A dense network of dry channels characterizes the area (Fig‐ ure 3). The topography of the area is formed in sandy deposits overlain by a loess layer that was deposited during the latest glacial period. Soils are therefore dominantly loess-derived luvisols, with their physical parameters related much more to land use than to soil texture (Van Oost et al., 2005).

Top soils have a very high silt percentage (on the average 75%) and moderate clay and sand content (on the average 11% and 14% respectively) (Van Oost et al., 2005).

The watershed land use is mainly agricultural. Forested (5%) and pasture (4%) zones cover the steep slopes as well as some of the thalweg areas. A built-up zone is located in northwestern part of the Ganspoel watershed and represents 9% of its area (Steegen et al., 2001). The main

**Figure 3.** Location and aerial view of the Ganspoel watershed.

#### *4.2.2. The hydrological database*

**Figure 2.** Location, contour map and hydrographic network of the Cannata watershed.

ed, rainfall spatial variability decreased on a monthly and yearly basis.

In the monitoring period of 1996 to 2003 the hydrological observations were collected utilis‐ ing the following equipment (Figure 2): a meteorological station (A, located outside of the watershed) recording rainfall, air temperature, wind, solar radiation and pan evaporation; two pluviometric stations (B and C); and a hydrometrograph (D) connected to a runoff wa‐

In the observation period yearly rainfall between 541 and 846 mm (mainly concentrated from September to March) was recorded at the station A, with a mean and standard devia‐ tion (SD) of 662 and 134 mm respectively. The corresponding yearly runoff was in the range 30.7 to 365.8 mm, with a mean of 105.3 mm and SD of 100 mm. The coefficient of yearly run‐ off, calculated as the ratio between total runoff and total rainfall as recorded by station A, varied between 5% and 41%, with a mean and SD of 15% and 75% respectively. Occasional high differences in recorded rainfall events between the three gauges were found; as expect‐

ter automatic sampler (E) for the measurement of sediment concentration in the flow.

*4.1.2. The hydrological database*

8 Research on Soil Erosion Soil Erosion

The climate of central Belgium shows relatively cool summers and mild winters resulting in an average annual temperature of 11 C. Annual precipitation varies normally between 700 and 800 mm year-1 and is well distributed over the year. High intensity rainfall events occur mainly in spring and summer: such thunderstorms may reach peak rainfall intensities of ca. 70 mm h-1 while total rainfall amounts may amount to 40 mm, exceeding rarely 60 mm.

**Event**

more details).

watershed (Ganspoel database, 2007).

**5. Model implementation**

sheds using the GIS interface incorporated into AnnAGNPS.

the Ganspoel watershed in 155 cells and 65 reaches (Figure 4b).

**Rainfall Runoff**

**depth duration**

**volume**

31/10-01/11/1998 25.0 19.3 1.67 6.7 0.064 6.9 58.9 14/11/1998 15.5 14.4 0.71 4.6 0.032 0.7 6.1 29/11/1998 18.5 19.9 0.56 3.0 0.025 1.4 12.0 07/12/1998\* 7.0 60.8 0.93 13.3 0.026 - - 19/12/1998\* 4.5 5.7 0.27 6.0 0.033 - - 07/01/1998\* 28.0 51.5 1.80 6.4 0.061 - - 16-17/01/1999 14.5 21.0 0.94 6.5 0.033 2.6 21.8 25/01/1999\* 21.5 49.5 1.61 7.5 0.788 - - 28/01/1999 8.0 3.8 0.71 8.9 0.046 3.0 25.6 07/02/1999 6.5 12.0 0.30 4.6 0.029 0.5 4.7 21/02/1999\* 8.0 49.5 2.36 29.5 0.768 - - 01/03/1999\* 6.0 8.1 1.29 21.5 0.777 - - \* Event not taken into account, because of inadequate sampling (see Van Oost et al., 2005 for

**Table 1.** Main characteristics of the observed events used for the AnnAGNPS model implementation at the Ganspoel

The watershed discretization into homogeneous drainage areas ("cells") and the hydro‐ graphic network segmentation into channels ("reaches") were performed for both water‐

The geometry and the density of the drainage network were modeled by setting the Critical Source Area (CSA) to 1.25 ha and the Minimum Source Channel Length (MSCL) to 100 m for the Cannata watershed, which allowed a suitable representation of the same watershed in a previous study (Licciardello et al., 2006). Such values were decreased to 0.5 ha and 50 m re‐ spectively for the Ganspoel watershed, because of its higher land use heterogeneity (Near‐ ing et al., 2005). The Cannata watershed resulted in 78 cells and 32 reaches (Figure 4a), while

The elevation GIS layer was arranged by digitizing contour lines every 2 m on a 5-m resolution DEM; land use and soil input data were derived from 25-m resolution GIS maps. The morpho‐ logic parameters (i.e., cell slope length and steepness) as well as the dominant land uses and soil types were directly associated with each drainage area by means of the GIS interface.

**Runoff coefficient**

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

**(mm) (h) (mm) (%) (m3 s-1) (Mg) (kg ha-1)**

**Peak**

**flow Sediment yield**

http://dx.doi.org/10.5772/50427

11

The hydrological database was collected during a recording period of about 2 years (May 1997-February 1999). The rainfall and flow/sediment measurement station was located at the outlet of the watershed. The rainfall events were recorded by a tipping-bucket rain gauge (logging interval equal to 1 minute with 0.5-mm tips). Water depths were continuously measured with a time interval of 2 minutes and an accuracy of 2 mm by a San Dimas flume equipped with a flowmeter, using a submerged probe level sensor. Water discharge was then calculated by a constant relationship between water depth and discharge. The suspend‐ ed sediment concentration, measured by an automated water sampler which a flow-propor‐ tional sampling rate (every 30 m3 runoff), was determined by oven-drying every sample at 105 C for 24 hours.

Seventeen runoff events, corresponding to rainfall depths in the range 5.5-57.5 mm, were ade‐ quately sampled (Table 1). The sampled events concerned generally low runoff volumes (15 with runoff depths lower than 2 mm), but the most intense event (13-14 September 1998) pro‐ duced a runoff volume of 9.5 mm. Event-based sediment yields were in the range 2 to 604 kg ha-1 (Table 1). Ten other events were not taken into account because of inadequate sampling.


Prediction of Surface Runoff and Soil Erosion at Watershed Scale: Analysis of the AnnAGNPS Model http://dx.doi.org/10.5772/50427 11


\* Event not taken into account, because of inadequate sampling (see Van Oost et al., 2005 for more details).

**Table 1.** Main characteristics of the observed events used for the AnnAGNPS model implementation at the Ganspoel watershed (Ganspoel database, 2007).
