GIS-based comparative analysis of potential soil erosion in two agricultural systems: Dry farming and center-pivot irrigation. A case study in Southern Portugal

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GIS-based comparative analysis of potential soil erosion in two agricultural systems: Dry farming and center-pivot irrigation. A case study in Southern Portugal

Materials and Methods
The potential soil erosion analysis was made for a 65 hectares circular area, located in a farm near the city of Évora (Southern Portugal), considering two possibilities:

A traditional rain-fed agricultural system with a wheat crop, considering the natural annual rainfall of the region, which is about 600 mm.
A corn crop irrigated with a 455-m center-pivot system, that applied, during the crop cycle, a total of 600 mm of water, with an application efficiency of 80 %.

The field area was subject to a topographic survey with a GPS system: Trimble RTK / PP - 4700, with a planimetric and altimetric precision of less than 0.02 m and 0.04 m, respectively. The grid model in the origin of the calculus made was built following the above steps:

Topographic survey with GPS,
Field point coordinates importation to "ArcView" software (ESRI, 1999).
With this points was then calculated a 5 m x 5 m grid.

The topography of the field area is undulated with a difference from the higher to the lower point of 25 meters, with slopes from 0 to 21 %, and 34,8 % of the area with a slope higher than 6 %.

The Wischmeier & Smith (1978) Soil Loss Equation:
A = EI30 K L S C P (1)
where:
A - average annual soil loss (10³ kg ha^-1)
EI30 - rain erosivity factor (MJ mm ha^-1 h^-1)
K - soil erodibility factor (10³ kg ha h ha^-1 MJ^-1 mm^-1)
L - slope length index
S - slope-steepness index
C - cover and management index
P - support practice index

was used to estimate soil loss.

For an annual natural rainfall of 600 mm, Marques da Silva (1999) obtained an average EI30 factor of 1600 MJ mm ha^-1 h^-1. The water kinetic energy from the center-pivot, needed to determine the EI30 factor, was calculated using Kincaid (1996) expression:
E_k =e₀ + e₁*R (2)
where E_k is overall drop energy (J kg^-1 = J mm^-1 m^-2), e₀ and e₁ are regression coefficients that are tabled (Kincaid, 1996) according to sprinkler type, and R (mm m^-1) is a parameter determined by:

where D is the nozzle diameter (mm), H is the nozzle pressure head (m) and e and f constants that depend upon sprinkler type. For spray sprinklers with serrated plates Kincaid (1996) gives the following values: e₀ = 6,2; e₁ = 0,45; e = 2,0 e f = 0,5.

The EI30 values determined for the different spans of the center-pivot lateral are presented in table 1.

**Table 1 -** EI30 values for the different center-pivot lateral spans
Lateral spans	Length	Number of Sprinklers	Average Nozzle size	R (Eq. 3)	E_k (Eq. 2)	E_k Crop cycle	Maximum application rate in 30 min *	EI30Crop cycle
	m		mm	mm m^-1	J mm^-1 m^-2	J m^-2	mm h^-1	MJ mm ha^-1 h^-1
1	50	17	2.1	1.2	6.7	5048	20	1010
2	50	16	3.7	3.7	7.8	5885	30	1765
3	50	17	4.7	5.9	8.9	6643	50	3321
4	50	16	5.5	8.1	9.8	7379	60	4427
5	50	17	6.2	10.3	10.8	8117	60	4870
6	50	16	6.9	12.7	11.9	8944	70	6261
7	50	17	7.5	15.0	13.0	9724	70	6807
8	45	15	8.0	17.1	13.9	10423	80	8338
9	45	16	8.5	19.3	14.9	11167	80	8934
10	15	5	8.5	19.3	14.9	11167	85	9492

* Values from Valadas (1997)
The K factor was determined using soil texture information and the methodology presented by Marques da Silva (1999), and the values obtained according to the different soil families, varied from 0.437 to 1.529 10-6 kg ha h ha-1 MJ-1 mm-1. The L factor was calculated with the digital elevation model of the field and the methodology presented by Jenson & Domingue (1988). The LS factor was then determined following McCool et al., (1987) methodology adapt to the grid form. The P factor was obtained according to Wischmeier & Smith (1978), considering that the tillage operations were made along the contour lines using the some agricultural implements in both irrigated and rain-fed system. The average C factor for corn was 0.55, and for wheat 0.59.

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