Soil Physical Properties Affecting Soil Erosion in Tropical Soils
Deyanira Lobo Lujan1
Facultad de Agronomia, Instituto de Edafologia, Universidad Central de Venezuela, Maracay, Venezuela
Lecture given at the College on Soil Physics Trieste, 3-21 March 2003
LNS0418021
Soil Physical Properties Affecting Soil Erosion 233
INTRODUCTION
The total vegetated land area of the earth is about 11,500 hectare. Of this, about 12% is in South America. Of this, about 14% is degraded area. Water erosion, chemical degradation, wind erosion, and physical degradation have been reported as main types of degradation. In South America water erosion is a major process for soil degradation. Nevertheless, water erosion can be a consequence of degradation of the soil structure, especially the functional attributes of soil pores to transmit and retain water, and to facilitate root growth. Climate, soil and topographic characteristics determine runoff and erosion potential from agricultural lands. The main factors causing soil erosion can be divided into three groups • Energy factors: rainfall erosivity, runoff volume, wind strength, relief, slope angle, slope length. • Protection factors: population density, plant cover, amenity value (pressure for use) and land management. • Resistance factors: soil erodibility, infiltration capacity and soil management. The degree of soil erosion in a particular climatic zone, with particular soils, land use and socioeconomic conditions, will always result from a combination of the above mentioned factors. It is not easy to isolate a single factor. However, the soil physical properties that determine the soil erosion process, because the deterioration of soil physical properties is manifested through interrelated problems of surface sealing, crusting, soil compaction, poor drainage, impeded root growth, excessive runoff and accelerated erosion. When an unprotected soil surface is exposed to the direct impact of raindrops it can produce different responses: Production of smaller aggregates, dispersed particles, particles in suspension and translocation and deposition of particles. When this has occurred, the material is reorganized at the location into a surface seal. Aggregate breakdown under rainfall depends on soil strength and a certain threshold kinetic energy is needed to start detachment. Studies on necessary kinetic energy to detach one kilogram of sediments by raindrop impact have shown that the minimum energy is required for particles of 0.125 mm. Particles between 0.063 to 0.250 mm are the most vulnerable to detachment. This means that soils with high content of particles into vulnerable range, for example silty loam, loamy, fine sandy, and sandy loam are the most susceptible soils to detachment. Many aspects of soil behavior in the field such as hydraulic conductivity water retention, soil crusting, soil compaction, and workability are influenced strongly by the primary particles. In tropical soils also a negative relation between structure stability and particles of silt, fine sand and very fine sand has been found, this is attributed to low cohesiveness of these particles. The ability of a structure to persist is known as its stability. There are two principal types of stability: the ability of the soil to retain its structure under the action 234 D.L. Lujan of water, and the ability of the soil to retain its structure under the action of external mechanical stresses. (e.g. by wheels). Both types of stability are related with susceptibility to erosion. The soil susceptibility to sealing and crusting has been related to different indices:
• Consistency index C5 – 10 Consistency Index C5 – 10 = (w5 – w10) where w5 and w10 are the water contents, in percentage of dry weight for which the two sections of a part of the soil in the Casagrande cup touch each other over a distance of 1 cm, after 5 and 10 blows, respectively. It has been found that stable topsoils have values of C5 – 10 > 3, whereas unstable soils have values less than 2.5.
• Wet–sieving The distribution of water stable aggregates is determined by wet-sieving. During wet-sieving, soil aggregates are submerged and gently sieved under water to characterize the aggregate resistance to breakdown.
• Absolute Sealing Index ASI ASI is the minimum value of the hydraulic conductivity in the seal formed by impact of water drops The test measures the changes of saturated hydraulic conductivity of the seal formed by raindrop impact obtained from a layer of soil aggregates of approximately 1.5 cm thick and 2 to 4 mm in diameter that receive a simulated rainfall during 60 minutes with an intensity around 90 mm/hour.
• Infiltrability of wet and more or less sealed topsoils A non-destructive method whereby small amounts of water is supplied by drip sources placed close to the soil surface without dissipation of kinetic energy and the area of saturated flux is measured when gravitational flow begins to prevail. Once a steady flow is apparently obtained, lateral flow as well as evaporative flux are neglected, with the relation between the source discharge rate Q and the equivalent radius r of the saturated patch written as 2 Q = pr qi where qi is the infiltration rate through the crusted topsoil. One of the processes that determine water erosion is infiltration. Hydrologically the infiltration process separates rain into two parts. One part stored within the soil supplies water to the roots of vegetation and recharges ground water, while the other part which does not penetrate the soil surface is responsible for surface runoff. On stable soils without gradual deterioration of the soil structure, the decrease of infiltration rate results from the inevitable decrease in the gradient of matric Soil Physical Properties Affecting Soil Erosion 235 potential, one of the forces drawing water into the soil, which occurs as the infiltration process proceeds. In unstable soils, a second cause for the decrease in the infiltration rate with time is the deterioration of soil structure on the surface occurring during the infiltration process. This deterioration may cause the formation of a dense crust, and a partial sealing of the profile. Runoff and soil loss prediction has been widely used as a tool to guide conservation planning. The prediction technology can be characterized as empirically based, process-based or a combination of the two. There are numerous process-based models. In a hydrologic simulation model to compute runoff and soil loss, relationships for fundamental runoff and soil loss processes should be combined with relationships for fundamental hydrologic processes. One of these models, SOMORE, accounts for infiltration of rainfall into the soil as limited by surface sealing effects and limited layers close to the soil surface, and by internal drainage or subsurface runoff as affected by rainfall infiltration, effective root depth and saturated hydraulic conductivity of the limiting soil layer. The SOMORE model uses as inputs: daily rainfall, daily evapotranspiration, infiltration rate, and rainfall intensity and soil conditions at the rooting depth, such as soil moisture at saturation, soil moisture at liquid limit, soil moisture at plastic limit, soil moisture at field capacity, soil moisture at -0.15 MPa, soil moisture at -1.5 MPa, soil moisture at the first day, and saturated hydraulic conductivity of the subsoil. Some outputs of model are: water losses by runoff or surface drainage (mm), waterlogging (mm), duration of waterlogging (hours), water losses by internal drainage (mm) and soil moisture at rooting depth (mm). The model also provides additional outputs, e.g. days with waterlogging, days with excessive soil moisture for tillage, days with excessive soil moisture for the crop, days with appropriate soil moisture for tillage for the crop, etc.
CASE STUDIES
Two case studies in Venezuela will be used to illustrate the role of soil physical properties in the erosion process. Two soils of gently rolling topography (5 – 10 % slopes) representating large rainfed agricultural areas were selected. The Barinas soil is located in the Western Plains. The Chaguaramas soil is located in the Central Plains. Thirty years ago, the land use changed from pasture and livestock to crops like maize and sorghum in the Western Plains, and to sorghum in the Central Plains, highly mechanized and livestock. Tillage operations previous to seeding, generally include two or three disc harrowing, oftentimes leaving a bare surface soil exposed to the rainfall impact during the first days of the crop. A modification of Fournier’s Index was used to assess the climatic aggressiveness 1 FI = p P ∑ i 236 D.L. Lujan where pi is monthly precipitation and P the annual precipitation. Correlations between monthly FI values and R factor values (Wishmeier’s method) are highly significant. Figure 1 shows the monthly Fournier Index distribution. For the Barinas soil the high values of Fournier Index (>7.5) occur from April to October, whereas for the Chaguaramas soil high values occur from May to August.
Barinas Chaguaramas
16 14 12 10 8 6 4 Fournier Index Fournier 2 0
b y l t ic an e a u go c ov J F Jun J O N D arch April M A Sept M Months
Figure 1. Fournier Index distribution
In Barinas most of the rainfall occurs from April through November (Figure 2). In Chaguaramas most of the rainfall (about 750 mm) occurs from May through October (Figure 3). In both regions, 50% of the rainfalls have intensities greater than 25mm/h. Soil Physical Properties Affecting Soil Erosion 237
Barinas
(1980 - 1999) 300 Total rainfall Rainfall > 25 mm/h
250
200
150 Rainfalll (mm) 100
50
0 J F M A M J J A S O N D Months
Figure 2. Total rainfall and rainfall with intensities higher than 25 mm.h-1 in Barinas
Chaguaramas
(1980 - 1999) 200 Total rainfall Rainfall > 25 mm/h
150
100
50 Rainfalll (mm)
0 J F M A M J J A S O N D Months
Figure 3. Total rainfall and rainfall with intensities higher than 25 mm.h-1 in Chaguaramas 238 D.L. Lujan
Table 1 shows the particle size distribution of two Venezuelan Alfisols. Both soils are sandy loam in the soil surface, but they differ in the proportion of silt and different sand sizes.
Table 1. Particle size distribution
Particle diameter (µm) Soil Depth < 2 2 – 50 50 – 100 – 250 – 500 – 1000 Textural 100 250 500 1000 –2000 Class 0-15 15 12.0 21.0 28.0 16.5 3.5 1.0 SL 15-35 18 10.5 19.5 25.0 22.0 6.5 1.5 SL Barinas 35-55 24 12.0 22.0 24.0 13.5 3.5 1.0 CSL 55-70 27 14.5 22.0 21.5 11.0 3.0 1.0 CSL 0-10 10 34.0 21.0 25.5 8.5 1 0 SL 10-18 14 37.5 23.5 18.5 4.5 1 1 L Chaguaramas 18-35 17 47.0 16.0 12.0 4.0 2 2 L 35-45 24 44.0 13.5 9.5 5.0 2 2 L 45-70 20 40.0 20.0 13.5 4.5 2 0 L
In the Chaguaramas soil particles between 2 and 250 µm prevail. In the Barinas soil particles between 50 and 500 µm prevail. Figure 4 shows the structure stability of Barinas and Chaguaramas soils assessed by means of wet-sieving (Yoder’s method). The Chaguaramas soil shows a low proportion of larger water stable aggregates and a high proportion of smaller water stable aggregates. The Barinas soil shows a higher proportion of large water stable aggregates. The maintenance of large aggregates stable in water on the surface soil is important to maintain high water intake rates and to reduce evaporation losses of the water stored in the soil.
> 0,8 0- 0,4 0,4 - 0,1 < 0,1 > 0,8 0,8 - 0,4 0,4 - 0,1 < 0,1 80 40
Chaguaramas Soil 60 30
40 20
20 10 Stable aggregates (%) Stable aggregates
Diameter of aggregates (mm) Diameter of aggregates
0 Figure 4. Stable aggregates distribution
Soil Physical Properties Affecting Soil Erosion 239
Figure 5 shows the behavior of saturated hydraulic conductivity obtained from a layer of soil aggregates of approximately 1.5 cm thick and 2 to 4 mm in diameter that received a simulated rainfall during 60 minutes with an intensity around 90 mm/hour. ASI is the Absolute Sealing Index which is equal to the minimum value of hydraulic conductivity in the seal formed by impact of water drops, Kws the hydraulic conductivity value with seal, Kns the hydraulic conductivity value without the seal and RSI the Relative Sealing Index which is the ratio Kns/Kws. The effect of raindrop impact on the hydraulic properties of the layer of soil aggregates is more evident in the Chaguaramas soil. This can be explained because the Chaguaramas soil shows a relatively high percentage of silt, fine sand and very fine sand. The Absolute Sealing Index in the Chaguaramas soil is very low - about 2.9 mm/h, and that in the Barinas soil is higher - about 8 mm/h. The amount of surface soil removed by runoff water depends to a large extent on the resistance of soil aggregates to be disrupted by the energy of raindrop impact. The ability of a surface soil to accept a continuous heavy rainfall is a critical factor in the prevention of accelerated erosion of soils, and is related to the stability of aggregates to raindrop impact and to the resistance to the shearing force of running water. Moreover, any dispersed clay may effectively block the pores between the micro-aggregates and give an extremely low infiltration rate. Dispersion and swelling of clay results in the elimination of the larger soil pores with a consequent reduction in soil hydraulic conductivity. Soil compaction, a process resulting in an increase in soil bulk density and a decrease in total pore space, significantly influences soil hydraulic properties (pore size distribution, water retention and hydraulic conductivity) as well as soil strength and mechanical impedance to root growth. The pore size distribution and continuity is related to water transmission, especially to the relative proportion of drainage pores. Figure 6 shows that a compacted layer is evident at 15 cm in the Barinas soil and at 10 cm in the Chaguaramas soil. Figure 7 shows the relative importance that changes in physical conditions on the surface or inside the profile could affect the infiltration process. In the Chaguaramas soil, the infiltration and water movement are more limited by a compacted layer close to the surface. In Barinas soil the surface seal is more limiting.
240 D.L. Lujan
Chaguaramas Soil 80 10 Kns 70 Kws 60 ASI = 2,9 mm/h 0 50 RSI = 7,8 ⇐ 22.6/2.9 K 40 mm/h 30 20 10 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Time (minutes)
Barinas soil
120 ASI = 8,2 mm/h 110 Kns 100 RSI= 6.7 ⇐ 55/8.2 Kws 90 80 K 70 mm/h 60 50 40 30 20 10 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Time (minutes)
Figure 5. Absolute and Relative Sealing Indices in Chaguaramas and Barinas soils
Soil Physical Properties Affecting Soil Erosion 241
Figure 6. Structural Indices
Figure 7. Infiltration rate and saturated hydraulic conductivity in the profile 242 D.L. Lujan
The most important effect of soil sealing is the reduction in infiltration rate shown in Fig. 8. However, infiltration may be also severely impeded by compacted layers, because they become limiting barriers for deep percolation and drainage of excess infiltrated rainfall, thus increasing the risks of waterlogging, water runoff losses and soil erosion.
300 Barinas Soil
250 200
150 i (mm.h -1) 100
50
0 0 50 100 150
140 Chaguaramas Soil 120 100 80 i -1 60 (mm.h ) 40 20 0 0 50 100 150
Time (minutes)
Figure 8. Infiltration rate (double ring method)
Estimates of water runoff losses during the growing period of sorghum was obtained using the hydrological process-based model SOMORE for the two conditions: 1) bare soil, and 2) rainy seasons of years with annual rainfall close to the average (return period of two years). Figure 9 shows the accumulated rainfall and runoff in the Barinas soil. The runoff after saturation is insignificant, because the compacted layer does not limit water movement. The total runoff is about 30%. In the Chaguaramas soil (Figure 10), Soil Physical Properties Affecting Soil Erosion 243 the compacted layer close to the soil surface and soil sealing affects the runoff. The total runoff is greater than 50%.
BARINAS 2000
1500 Rainfall
1000 (min) 500 Surface runoff
Runoff after 0 saturation 1 21 41 61 81 101 121 141161 Time (days)
Figure 9. Accumulated rainfall and runoff in the Barinas soil
CHAGUARAMAS 1000 Rainfall 800
600
(min) 400 Total runoff Surface runoff 200 Runoff after saturation 0 1 24 47 70 93 116 139 162 Time (days)
Figure 10. Accumulated rainfall and runoff in the Chaguaramas soil