Comparative analysis of potential soil erosion in dry farming and centre-pivot irrigation

Comparative analysis of potential soil erosion in dry farming and centre-pivot irrigation


José Rafael Marques da Silva* and Luís Leopoldo Silva**
Department of Rural Engineering, University of Évora
Apartado 94, 7002-554 Évora, Portugal
* [email protected]  , ** [email protected] 

The use of low pressure center-pivot systems, with high water application rates, to irrigate soils with low permeability often leads to runoff and soil erosion problems; sometimes grater than those due to natural rainfall.

The use of GIS tools and the Soil Loss Universal Equation can allow to study these affects for any particular conditions (soil, crops, topography) aiding the choice of a system that could minimize soil erosion by water.

In this study was showed that a low-pressure center-pivot irrigation system causes much more potential erosion that the traditional dry-farming systems. And its implementation without a previous environmental impact study can hypothecate the sustainability of soils at a medium/long run.

In order to increase productivity and profit, many farmers in Southern Portugal are changing their traditional agricultural dry-farming systems to irrigation. This change can have some environmental impacts in the region since most areas have low permeability soils and undulated lands, which presents some limitations to the proper management of some irrigation systems.

One of the first choices for farmers as been center-pivot systems, because they can be adapt to many different conditions (soils, crops, topography) and are very easy to automate. In order to reduce costs they try to irrigate great areas with only one machine, and use low-pressure systems with spray type sprinklers, which are less energy consumptive.

These low pressure systems are characterized for having great application rates that can vary from 150 to 300 mm h-1 at the end of a 400 meters lateral (Addink et al., 1983), which many times exceeds the infiltration capacity of soils leading to runoff (Beck & DeBoer, 1992) and soil erosion.

Associated with these high water application rates there is also a high water kinetic energy applied to the soil due to water droplet impact, which is directly related with soil crust formation, responsible for a significant infiltration reduction (Thompson & James, 1985), favoring runoff and erosion. Sharma et al. (1991) showed that soil detachment, the first step for crust formation, increases with the water kinetic energy.

Recent research (Ferreira, 2001) also showed that, in our conditions, erosion due to irrigation systems can be of the some or higher magnitude than erosion due to natural rainfall. Since great areas of Southern Portugal have poor soils the impact of introducing agricultural systems that could increase soil loss is something to consider. Soil losses occurring year by year have a great effect in soil productivity that can only be partially compensated with the application of more fertilizers, increasing crop costs and pollution risks.

This paper objective is to compare, for the same area, potential soil erosion from a traditional rain-fed wheat crop system and a center-pivot irrigated corn crop, using GIS tools and the Soil Loss Universal Equation.

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:

  1. A traditional rain-fed agricultural system with a wheat crop, considering the natural annual rainfall of the region, which is about 600 mm.
  2. 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)
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:
Ek =e0 + e1*R             (2)
where Ek 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) Ek (Eq. 2) Ek 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.

In Figure 1, the rain-fed system, is possible to see that in an average year this field could be classified in general as a low risk area for soil erosion (<1.0 103 kg ha-1), with the exception of same small areas where the risk could be classified as small to moderate (1.0 – 2.0 103 kg ha-1) or moderate to high (2.0 – 6.0 103 kg ha-1).

Figure 1 – Potential erosion values for the rain-fed system.
For the same field considering the irrigated production system (Figure 2) the erosion risk is higher. The area classified as low risk area is less and there are even some areas with high to severe erosion risks (> 6.0 103 kg ha-1).

Figure 2 – Potential erosion values for center-pivot irrigation.
The difference from these two situations is mainly due to the different water application, which in the second case is made with higher application rates, leading to higher erosivity factors. The EI30, which increases along the center-pivot lateral (Table 1), can achieve, after the third span (150 m length) values higher than those from natural rainfall, even in severe years.

With application rates so high and soils with low infiltrability, and sloppy areas, it’s easy to accept that the impact of using low-pressure center-pivots could be high. Taking soil conservation as a priority, a preliminary study could determine that the study area was not adequate for an irrigation system with the characteristics of the one studied. The alternative would be the use of a system with lower application rates, like center-pivots with high pressure rotating sprinklers, solid-set sprinkler systems or even trickle irrigation, which could irrigate the field applying lower water application rates and thus having less erosive power.

The use of GIS tools can play an important role in the evaluation of irrigation systems impact, allowing to estimate their erosion risks.

For the same area a center-pivot irrigation system could be much more erosive than the natural rain, especially if it’s a low-pressure system working in sloppy fields with low infiltrability soils.


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