Effects of management on rainfall infiltration and runoff from an Alfisol in semi-arid tropical India.

Introduction:

Alfisols of the Semi-Arid Tropics (SAT) are often characterized as structurally unstable, which is a result of low clay content, inactivity of the prevailing clay minerals and the tendency to stabilize only low amounts of decomposed organic matter. The consequence of poor and unstable soil structure is susceptibility to form surface crust or seal with a resultant decrease in infiltration and increase in runoff. Loss of rainwater as runoff not only limits the water available for crop production but also is an erosion hazard. To make Alfisols more productive, soil management practices should be used that improve soil structure and, thereby, infiltration of rain water into the soil and water available for use by crops.

A number of management options have been tried to conserve water in the soil, improve structural stability and increase productivity. The available management options can be grouped into three categories:

a. Tillage based systems
b. Organic systems
c. Biological systems

All these options were subjected to a detailed investigation in a long-term study at International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, India in collaboration with Queensland Department of Primary Industries (QDPI), Australia.

The trial was established in July 1988 on the research farm of ICRISAT located at Patancheru (180 N and 780 E), 26 km north west of Hyderabad, Andhra Pradesh, India on a soil that belongs to Patancheru series, a member of the clayey skeletal, mixed, isohyperthermic family of udic rhodustalfs (Figure 1).



Figure 1. The trial site


The surface is a sandy loam with increasing clay content up to a depth of 50 cm. Presence of gravelly murrum layer (a layer of decomposing parent material) at depths varying between 30 to 100 cm is an important feature of these soils. The experimental site has an average annual rainfall of 784 mm, with over 80% falling between the months of June and October.

The main experiment consisted of fifteen treatments, viz., nine treatments made up of three levels each of tillage and organic amendments in a factorial arrangement plus an additional six perennial-pasture treatments. The tillage depths were 0 (ZT), 10 (ST) and 20 (DT) cm. The three levels of organic amendments were nothing (B), farmyard manure @15 t/ha/yr (F) and rice straw @5 t/ha/yr (S). The six perennials were sole pigeonpea (P), sole Cenchrus ciliaris (C), sole Stylosanthes hamata (St), and mixtures of these species viz. PSt, PCSt and CSt.

Between 1988 and 1991 the tillage and mulch treatments were cropped annually to cereal crops. In 1992, perennial crops were removed and all treatments including those previously under perennial crops were planted to a cereal crop to compare the cumulative effects of different soil management histories. During 1995 further application of amendments was stopped in the treatments that were previously under deep tillage and receiving farmyard manure and rice straw amendments. The crop sequence was millet (BK 560) (1988) and a two-year rotation of sorghum (CSH9) and maize (Proagro 3448) between 1989 and 2000.

 

Tillage based systems:

Tillage is by far the most commonly adopted practice to break the surface seal and is often considered as essential soil management practice for Alfisols to restore the infiltration rates. This is generally carried out during the off-season (summer tillage) or with the first rains after the onset of monsoon. On Alfisols benefits of tillage on infiltration were found to be related to the rainfall since tillage. Each year runoff declined to a minimum following the tillage operation and reverted to that of untilled system after a few storms (Figure 2).

Figure 2. Effect of tillage on runoff

In 1989 most of the benefit from tillage on increased infiltration rates was lost during a single storm of 115 mm of rain 5 days after tillage. In 1993, 1994 and 1997 tillage effect on infiltration rate lasted for more than 60 days. Most rainfall events during these 2 years were smaller than 20 mm and the crop protected the surface before the crust was fully developed. Although the period that tillage reduced runoff varied from year to year, the runoff from tilled and zero tilled systems was similar after approximately 150 mm rain after the soil was tilled. The decline in infiltration rate from tillage was found to have the following exponential relationship with cumulative rainfall since tillage.

It= Ii + (Ii - If) exp (-bSR)

Where,

It = Infiltration rate at time t (mm/hr)

Ii = Initial infiltration rate (610 mm/hr)

If = Final infiltration rate (9.6 mm/hr)

SR = Cumulative rainfall since tillage

b = Empirical constant (0.0757)

The temporal variations in runoff as percent of untilled system indicate no significant difference in the annual runoff due to tillage (Figure 3).

Figure 3. Changes in runoff under shallow and deep tillage systems

However, a 10-20% reduction in annual runoff was recorded with deep tillage. The shallow tillage, which is similar to that practiced by the farmers of this region showed smaller benefit compared to that of deep tillage.

 

Organic systems:

Addition of organic amendments such as farmyard manure (FYM) and crop residues were reported to increase and/or maintain higher infiltration rates either by protecting the surface from rain drop impact or by improving the structural stability. In India FYM is the most common organic amendment. Application of FYM significantly reduced runoff compared to the un- amended system from the second year. Runoff was reduced to 40% over a period of six years with continuous application of manure (Figure 4).

Figure 4. Changes in runoff under straw and FYM systems

However, benefits from straw cover on runoff reduction are visible from the first year with a cumulative benefit. Runoff from straw mulched treatments was reduced from about 60% of the unamended system in the first year to 20% in the fourth year. The average infiltration rate derived from rainfall runoff hydrographs was 18.4 to 19.8 mm/hr with manure and 30.4 to 31.1 mm/hr under straw mulch. However, much of this benefit was lost during the first two years after stopping further application of amendments to the system.

Biological systems:

Biological systems involve use of crops or trees for ameliorating soil structure. They protect the soil surface by providing a cover for extended periods and improve soil structure through the activity of roots and addition of litter. In case of legumes additional benefit includes soil fertility improvement through fixation of atmospheric nitrogen. Significant improvement in soil structure and infiltration was observed in this trial where six different combinations of cenchrus, stylo and pigeonpea were evaluated. Runoff from these systems was on par with the untilled bare plot during the first year (Figure 5).

Figure 5. Changes in runoff under perennial systems

After 4 years, runoff was negligible especially from the systems having stylo as sole or in the mixture. This benefit was found to be longer lasting compared to that obtained with organic amendments. The differences were significant during the fist four years after the systems were retuned to annual cropping. Analysis of rainfall-runoff hydrographs indicated an average infiltration rate of 23.7 to 36.4 mm/hr for different systems.

Implications for management

Increasing the amount of soil water available to the plants can increase crop production on alfisols in semi-arid tropics.

Rainfall is the only source of water and loss of rainwater, as runoff needs to be minimized. Runoff occurs whenever rainfall intensities exceed infiltration rate of the soil and surface storage.

The rainfall amount in excess of a given intensity was calculated form 5 min totals. The results are plotted in Figure 6,

Figure 6. Distribution of rainfall in excess of a given intensity

Which is a plot of the percent of total rainfall that occurred in excess of a given intensity. Thus, for example in 1989, 45% of the total amount of rain exceeded an intensity of 10 mm/hr and 10% was above an intensity of 50 mm/hr. This can now be used to estimate the infiltration rate required to limit the runoff losses to a desired level and to identify the management practice with which it can be achieved. From the figure it can be deduced that to limit the runoff losses to 10% of the rainfall an infiltration rate of 32 mm/hr is essential. Our studies with different systems indicate that such high rates of infiltration can be achieved under perennial vegetation or with straw mulching. Further reductions in runoff can be achieved by increasing the surface storage.

Conclusion:

The results of this long-term trial on Alfisol management indicate that effectiveness of management practices in reducing runoff depends on the ability to reduce the formation of crusts or seals and improve infiltration. The conventional practice of tillage to mechanically break the crust has little long-term effect in increasing infiltration rate, as the benefit of this practice is soon lost with the formation of a surface crust after a few rainfalls. The addition of amendments offers a sustainable way to improve infiltration rates but is constrained by the availability of the material and demand for other uses like fuel and fodder. Systems like perennial-annual rotations and agro-forestry hold promise especially to the farming systems that have livestock as a component. There is a need to develop alternative soil management systems that promote use of organics as soil amendments, which are essential to improve the structural stability and to maintain high rates of infiltration in Alfisols.

For more information please contact:

Dr KPC Rao
Scientist (Soil Science),
ICRISAT-Patancheru 502 324.