DERIVATION OF PLANT GROWTH COEFFICIENTS FOR THE USE IN WIND EROSION MODELS IN ARGENTINA

Abstract

Relationships between wind erosion soil loss ratio (SLR, the quotient between the soil loss in a ground cover and a bare and smooth soil) and the percent of soil coverage with plant residues or canopy have been mostly obtained by means of wind tunnel experiments where fluid-dynamic parameters, driven in the nature by climatic conditions, can be maintained constant. To test the behavior of SLR under natural conditions, we compared wind erosion measured in the field in a semiarid environment of Argentina, during 3 sunflower (Helianthus annus) and 3 corn (Zea mays) growth periods, with wind erosion calculated with available equations. Results showed that the relationship between measured SLR and percentage of soil cover with flat residues fitted well to the already available equation SLR f =e -a(SC) , where SC is the soil cover with flat residues and a is a constant, but with an a coefficient of 0.0605 instead of the originally provided 0.0438. This resulted in an averaged difference in the SLR of 37% between both equations. The variation in SLR was attributed to differences in the highest speeds used for the derivation of the original a coefficient (16 m s -1 ) than wind speeds occurring during field measurements in this study (10.8 m s -1 , in average). The relationship between SLR and soil coverage with flat residues for storms with erosion amounts higher than 100 kg ha -1 had an a coefficient of 0.039, very close to the original a coefficient. Measured SLR as a function of soil cover with corn and sunflower canopy was quite similar to calculations made with the previously available equation SLR c =e - 5.614(CC 0.7366) where cc is the fraction of soil surface covered with crop canopy. The published equation CC = e pgca+(pgcb.Pd-2 ), where pgca and pgcb are constants and Pd the days after seeding, was not adequate to explain the evolution of the percentage of soil cover by the crops. This equation was replaced by CC = a/(1+be -cx ), where a, b, and c are constants and x is the days after seeding. SLR calculated on the basis of field measurements was, as a function of the days after corn seeding, lower than SLR calculated with available equations at early-crop growth stages and higher at late-crop growth stages. At early-crop growth stages, a critical period for wind erosion occurrence due to the low soil coverage with plants, sunflower had a better wind erosion control efficiency than corn. Sunflower also increased its wind erosion control efficiency with favorable climatic conditions, whereas corn efficiency remained unchanged. Such differences were attributed to the canopy leaf arrangement of each crop (planophyles in sunflower and erectophyles in corn), which resulted in a more effective reduction of wind speed by sunflower leaves than by the narrow leaves of the corn at same growth stages. On the other hand, sunflower had a more efficient use of the solar radiation and a faster canopy growth. We conclude that the equations developed here for use in empirical wind erosion prediction models produce reliable results, even under variable climatic conditions. Such models are useful for sites like under variable climatic conditions. Such models are useful for sites like the semiarid Pampas, where detailed climatic information is lacking.