Donald Gabriëls

Rainfall erosivity in Cape Verde

This paper presents rainfall erosivity values derived from a 7-year rainfall recording in the Cape Verde islands, Central East Atlantic. The data set consisted of 63 storm events, continuously registered in 15-min intervals. Kinetic energy of storm rainfall corresponded to established values in other tropical locations. Two algorithms to estimate erosivity, expressed as energy times intensity, using daily rainfall or storm depth and duration as predictor variables are derived. Erosivity of design storms for various return frequencies is calculated for some locations in Santiago island. An indicative range for the annual rainfall erosion R-index is given. Data analysis further showed the extreme seasonal concentration of precipitation and erosivity at this location, with a very high fraction of total annual erosivity contained in the annual maximum 24-h rainfall.

Assessment of USLE cover-management C-factors for 40 crop rotation systems on arable farms in the Kemmelbeek watershed, Belgium

In order to calculate the actual erosion according to the universal soil loss equation (USLE) and to estimate the impact of land use on soil erosion it is important to know the C-factor. Based on the USLE crop-growth stages, the cover-management C-factors were calculated for the main crop rotation systems on arable farms in Belgium with particular reference to those occurring on 40 farms on silt and silt loam soils in the Kemmelbeek watershed, West-Flanders, Belgium. The different rotation systems were composed of the following crops: sugar beet (SB), winter wheat (WW), potato (PO), maize (M), beans (B), winter barley (WB), peas (PE), chicory (CH), leek (L), carrot (CA), celeriac (CE), rye grass (RG) and temporal grassland (TG). The erosivity factor R during the rotation was calculated on the basis of a detailed rainfall time series over a time period of 27 years. The crop data (development of cover, sowing and harvesting date, amount of residue) were collected from literature and personal communications. For most rotation systems the C-factors were between 0.28 and 0.38. High C-factor values were obtained with M/M/M/PO (C=0.47), and with CE/L/PO (C=0.51). Remarkable small C-values (C=0.24–0.27) were found when winter cereals (WW and WB) were put in the rotation scheme. Whether or not the foliage of the sugar beets was removed, had no effect on the C-factor. The effect of green manuring on the C-factor in some rotation schemes was smaller than could be expected. The C-factor was also affected by the position of the crop in the rotation scheme. The calculated C-factors indicated that the application of some rotation systems might cause more erosion by runoff. Therefore, these calculated C-factors can be used as a criterion to select an appropriate rotation system to reduce erosion risk on site. However, validation of these values using experimental field plot data is still required.

Using the seasonal and temporal precipitation concentration index for characterizing the monthly rainfall distribution in Spain

Summary The concentration of the rainfall in a year is an important aspect of the climate. An unbalanced distribution of rainfall evokes periods of rainfall excess and periods of drought which make plant and crop growth difficult. Rainfall concentration needs to be considered in the assessment and the prediction of soil losses by water erosion. A statistical derived index, known as the Precipitation Concentration Index is presented to be used in quantifying the relative distribution of the rainfall patterns. Based on the available data sets, two different calculation procedures were used for deriving the Precipitation Concentration Index (Seasonal and Temporal). The application of the two procedures on the rainfall data of two meteorological stations from different climatological regions revealed how the two methods can be used in quantifying the seasonal and temporal concentration of the rainfall. The rainfall data from meteorological stations situated in a central transect of Spain were used in this investigation. From the observed results, it can be concluded that in the Southern part of the transect, the concentration of the rainfall is not a seasonal effect (not strongly confined by a typical dry or wet period) but a temporal effect (bounded to the unreliable, unpredictable character of the rainfall).

Assessing the drop size distribution of simulated rainfall in a wind tunnel

Abstract The installation of a rainfall simulator inside the wind tunnel of I.C.E. (International Center for Eremology, Ghent, Belgium) enables to study the combined effect of wind and rain on the erosion processes. Simulated rainfall characteristics assessed were: the spatial distribution of the intensity in the working area of wind tunnel and the drop size distribution under different operating pressures and wind speed. The rainfall intensities varied between 37.9 mm h −1 to 143.0 mm h −1 depending on the number of operating nozzles, the applied pressure and the wind speed. Areas for different rain intensities with a uniformity coefficient higher than 80% were determined in the working area of the wind tunnel. Under simulated rainfall the raindrop diameter varied between 0.2 and 3 mm and this was affected by the operating pressure and by the wind speed. A narrower raindrop distribution and a smaller sorting coefficient, ( D 75 − D 25 )/ D 50 , was obtained under wind-driven rain (oblique rain) compared to vertical rainfall (without wind).

Raindrop-induced and wind-driven soil particle transport

Abstract A wind tunnel study under wind-driven rains was conducted to determine the combined effect of rain and wind on the rainsplash transport process. The rains driven by horizontal wind velocities of 6, 10 and 12 m s−1 were applied to three agricultural soils packed into a 20×55-cm soil pan placed at both windward and leeward slopes of 4.0°, 8.5° and 11.3°. Transport rates were measured by trapping the splashed particles at set distances in the upslope and downslope directions, respectively, for windward and leeward slopes. Rainsplash transport under wind-driven rains was adequately described (R2=0.93) by relating the transport rate to the rain impact pressure and wind shear velocity by log–linear regression technique. Average trajectory of a raindrop-induced and wind-driven particle was also adequately predicted by the momentum loss per unit time per unit length of travel (u*2/g). The travel distance is found to be three times greater than the path of a typical saltating sand grain.

Seasonal Predictability of Daily Rainfall Characteristics in Central Northern Chile for Dry-Land Management

Abstract The seasonal predictability of daily winter rainfall characteristics relevant to dry-land management was investigated in the Coquimbo region of central northern Chile, with focus on the seasonal rainfall total, daily rainfall frequency, and mean daily rainfall intensity on wet days at the station scale. Three approaches of increasing complexity were tested. First, an index of the simultaneous El Nino–Southern Oscillation (ENSO) was regressed onto May–August (MJJA) observed precipitation; this explained 32% of station-averaged rainfall-amount variability, but performed poorly in a forecasting setting. The second approach used retrospective seasonal forecasts made with three general circulation models (GCMs) to produce downscaled seasonal rainfall statistics by means of canonical correlation analysis (CCA). In the third approach, a nonhomogeneous hidden Markov model (nHMM) driven by the GCM’s seasonal forecasts was used to model stochastic daily rainfall sequences. While the CCA is used as a downsc...

Effects on pesticide spray drift of the physicochemical properties of the spray liquid

This research was on the effect of the physicochemical properties of the spray liquid on pesticide spray drift. Ten pesticide spray liquids with various physicochemical properties were selected for study. Some of these spray liquids were also examined with the addition of a polymer drift-retardant. In the first part, laboratory tests were performed to measure surface tension, viscosity, evaporation rate and density of the spray liquids. Subsequently, drift experiments were performed in a wind tunnel. From the results it was found that the dynamic surface tension is a major drift-determining factor, and also that the addition of a polymer drift-retardant can reduce drift significantly by increasing the viscosity. Drift reduction was found to be less effective with spray liquids of emulsifiable and suspendable formulation types than with spray liquids of water-dispersible granules and powders.

The effect of slope length on the amount and size distribution of eroded silt loam soils: short slope laboratory experiments on interrill erosion

Abstract The data of rainfall simulation experiments on a sandy loam and a loamy sand were compared to assess the effect of slope length on the soil loss from short slopes (between 0.3 and 0.9 m). The total soil loss and the aggregate size distribution in the total soil loss were determined after 90 min of rainfall with rainfall intensities between 22.0 and 78.5 m h −1 . The experimental results indicated that the slope length effect is different for the two soils. Although for both soils the total soil loss increased with increasing slope length, the soil loss per unit slope length increased with increasing slope length only for the sandy loam (soil 1) and decreased with increasing slope length for the loamy sand (soil 2). The difference in slope length effect between the two soils was also reflected in the aggregate size distribution in the soil loss. The different aggregate sizes displayed different relationships between soil loss and slope length. For the loamy sand the slope length effect was less for the large aggregate sizes which contained more sand than the original soil. For both soils the slope length effect was most pronounced for the aggregate sizes smaller than 0.05 mm.

THE EFFECT OF WIND ON RAINDROP IMPACT AND RAINSPLASH DETACHMENT

In wind–driven rains, variations in raindrop trajectory and frequency are expected due to the changes in the angle of raindrop incidence. This article presents experimental data on the effects of horizontal wind velocity on rainsplash detachment. In a wind tunnel facility equipped with a rainfall simulator, windless rains and rains driven by horizontal wind velocities of 6, 10, and 14 m s–1 were applied to three agricultural soils packed into 20 U 55 cm soil pans with both windward and leeward slopes of 7%, 15%, and 20%. Rain intensity was directly measured with inclined raingauges oriented with respect to the prevailing wind direction. These measurements showed that the actual amount of rainfall intercepted on the soil surface varied widely depending on the angle of rain incidence, which was a function of the rain inclination and slope gradient and aspect. A two–dimensional numerical model was used to estimate wind–driven raindrop trajectories. Rain energy was also measured by a kinetic energy sensor. Theory and measurement showed that an exponential relationship existed between the energy of simulated rainfall and the applied horizontal wind velocity. The experiments led to the conclusion that the wind not only increased the resultant raindrop impact velocity but also altered the angle of raindrop incidence, resulting in variable raindrop impact frequency and impact angle. Accordingly, differential rainsplash detachment occurred depending on the changes in raindrop trajectory and frequency with wind velocity and direction. There were significant differences in the detachment rates between the aspects, and the rates were as much as 44 times greater in the windward slopes than in the leeward slopes.

Heavy metal transport from dredged sediment derived surface soils in a laboratory rainfall simulation experiment.

Heavy metals in dredged sedimentdisposal sites may be transported through runoff andpercolation. In the present study metal fluxes byrunoff and percolation were determined under simulatedrainfall at a slope of 19% and a rainfall intensityof approximately 40 mm h-1. These circumstancescorrespond to half the mean annual erosivity of rainunder Belgian weather conditions. Surface runoff andpercolating water samples were analysed for suspendedsolids, total dissolved carbon and Cd, Cr, Cu, Ni, Pb and Zn.Runoff rate and sediment yields were highest for asilt loam sediment, characterised by a low clay andorganic matter content. Metal concentrations in runoffand percolating water varied widely between thesediments studied and were related to the total metalcontent in the sediment. In runoff and percolatingwater from the contaminated sediments, metalconcentrations strongly exceeded the Netherlands Areference values for ground water quality. Very highmetal fluxes were observed for the recently oxidiseddredged sediment. Metal transport per unit surfacearea through percolating water was from two to morethan twenty times greater than that in surface runoff.