Papers by Donald Gabriels
Sociedade & Natureza
Steeplands, when cleared from forests, are susceptible to erosion by rainfall and are prone to la... more Steeplands, when cleared from forests, are susceptible to erosion by rainfall and are prone to land degradation and desertification processes. The dominant factors affecting those erosion processes and hence the resulting runoff and soil losses are the aggressiveness of the rainfall during the successive plant growth stages, the soil cover-management, but also the topography (slope length and slope steepness). Depending on the type of (agro) climatological zone, the runoff water should either be limited and controlled (excess of water) or should be enhanced and collected from the slope on the downslope cropping area if water is short (negative soil water balance). Examples are given of practical applications in Ecuador where alternative soil conservation scenarios are proposed in maize cultivation in small fields on steep slopes. Adding peas and barley in the rotation of maize and beans resulted only in a slight decrease of the soil losses. Subdividing the fields into smaller parcel...
SN Applied Sciences
In tropical small islands the application of hydrological modelling is challenged by the scarcity... more In tropical small islands the application of hydrological modelling is challenged by the scarcity of input data. Using in-situ and statistically estimated data, a hydrological model was calibrated and validated for the Upper Navet watershed in Trinidad, a small Caribbean island. The model was built using the soil water assessment tool (SWAT). The sensitivity analysis, calibration and validation were performed in SWAT calibration and uncertainty program (SWAT-CUP) using sequential uncertainty fitting (SUFI-2). The results revealed that for the estimated volume of water flowing into the reservoir (Flow_In) there were six sensitive parameters. To estimate the reservoir volume (Res_Vol), a modification of only the effective hydraulic conductivity was required. The model’s performance for the Flow_In validation showed acceptable values (R2 = 0.91 and NSE = 0.81). The uncertainty analysis indicated lower than recommended values for both the R-factor (0.46) and P-factor (0.31). For Res_Vol...
Drylands can have between 35% and 95% of deficit of water in their soils. If a permanent pluviome... more Drylands can have between 35% and 95% of deficit of water in their soils. If a permanent pluviometric deficit occurs, this phenomenon is called ‘aridity’, while ‘drought’ is related to a temporary deficit. Drylands are also affected by several types of irregular droughts. Desertification is more related to drought than to aridity. Combating desertification is managing drylands in view of their preservation or restoration. The SUMAMAD (Sustainable Management of Marginal Drylands) project was implemented by UNESCO’s Man and the Biosphere (MAB) Program together with the United Nations University Institute for Water, Environment & Health (UNU-INWEH), and funded by Flanders, Belgium (2008-2014). The management of drylands was mainly performed at sites at UNESCO-MAB Reserves, and included: A solar desalinization plant was built in the Omayed Biosphere Reserve in Egypt. Chicken farming in compacted grassland was carried out in the Hunshandake sand area of China. Deficit irrigation for quin...
SN Applied Sciences, 2019
Small island states are vulnerable to changes in extreme weather events. In order to mitigate the... more Small island states are vulnerable to changes in extreme weather events. In order to mitigate the effects of these events such as droughts, the understanding of the patterns of meteorological droughts is one of the first steps. This study aims to build a baseline scenario of meteorological droughts using the Standardized Precipitation Index (SPI) for the southernmost Caribbean islands of Trinidad and Tobago. Fourteen precipitation datasets, for the period 1980 to 2014, were analyzed to identify and assess temporal and spatial variations of droughts. The relevant SPI temporal intervals for the area are 2-M (Month), 5-M Dry and 7-M Wet corresponding to the dry and wet seasons and 12-M. Meteorological droughts were found to be heterogeneous in occurrence, magnitude, severity and frequency across Trinidad. Meteorological droughts were identified in the following years; for the 2
PLOS ONE, 2016
The research on wind-driven rain (WDR) transport process of the splash-saltation has increased ov... more The research on wind-driven rain (WDR) transport process of the splash-saltation has increased over the last twenty years as wind tunnel experimental studies provide new insights into the mechanisms of simultaneous wind and rain (WDR) transport. The present study was conducted to investigate the efficiency of the BEST ® sediment traps in catching the sand particles transported through the splash-saltation process under WDR conditions. Experiments were conducted in a wind tunnel rainfall simulator facility with water sprayed through sprinkler nozzles and free-flowing wind at different velocities to simulate the WDR conditions. Not only for vertical sediment distribution, but a series of experimental tests for horizontal distribution of sediments was also performed using BEST ® collectors to obtain the actual total sediment mass flow by the splash-saltation in the center of the wind tunnel test section. Total mass transport (kg m-2) were estimated by analytically integrating the exponential functional relationship using the measured sediment amounts at the set trap heights for every run. Results revealed the integrated efficiency of the BEST ® traps at 6, 9, 12 and 15 m s-1 wind velocities under 55.8, 50.5, 55.0 and 50.5 mm h-1 rain intensities were, respectively, 83, 106, 105, and 102%. Results as well showed that the efficiencies of BEST ® did not change much as compared with those under rainless wind condition.
Soil erosion by water is a severe problem in the eastern loess belt of Northern China, particular... more Soil erosion by water is a severe problem in the eastern loess belt of Northern China, particularly on agricultural land. The erosion problems are to a large extent associated with improper soil tillage practices. Changing the current tillage practices could therefore reduce soil loss. This will, however, also affect the water balance and hence the available water for crop growth, particularly in dryland farming systems. A field study was carried out on five plots on a slope field near Luoyang, Henan province, China, in order to compare the water balance under different soil tillage practices. The terms of the water balance were determined from August 1999 until April 2001. From October to June all plots were under winter wheat. After harvesting, four soil tillage practices were applied: conventional tillage, no tillage, subsoiling and reduced tillage. A two-crop rotation of winter wheat and summer corn or peanuts was applied as well. Analysis of the different components of the soil water balance enabled to determine the most suitable practice for crop growth. The results, though preliminary at this stage of the study, show that subsoiling resulted in the highest increase in moisture storage and lowest evaporation during the fallow period. The two-crop rotation with peanuts also showed promising results, mainly due to decreased evaporation of the soil surface. The no-tillage and conventional tillage gave intermediate results, whereas the reduced tillage was the worst alternative.
In order to increase dryland crop productivity, water harvesting techniques (WHT) have received r... more In order to increase dryland crop productivity, water harvesting techniques (WHT) have received renewed attention, leading to their massive implementation in marginal drylands. However, versatile tools to evaluate their efficiency under a wide range of conditions are often lacking. For two case studies in the arid and semi-arid central-northern zone of Chile, a fully coupled 3D surface-subsurface hydrological model based on the Richards' and the Saint Venant equations was used to evaluate and improve existing water harvesting techniques using infiltration trenches (locally called 'zanjas'). The model was parameterized with detailed runoff and soil-water content data collected during simulated rainfall from a 6 x 2 m experimental plot including a catchment area and infiltration trench at the arid site. Using seven responsive parameters identified by a global sensitivity analysis, surface and subsurface flow processes were calibrated simultaneously. The calibrated model ac...
Land Degradation & Development, 2014
Studies in arid regions have shown that scattered trees play an important role in reducing the ne... more Studies in arid regions have shown that scattered trees play an important role in reducing the negative effects of climate and soil aridity. They strongly influence the environmental conditions under their canopies. Scattered trees often function as 'fertility islands', in that they provide favourable conditions for the recruitment of other plants. The most critical factor controlling plant productivity in arid regions is water availability in the soil. Hence, understanding the soil-water relationships of nursed ecosystems is of crucial importance. The objective of this study was to investigate the facilitating role of Acacia trees on the herbaceous layer in a forest steppe ecosystem in the Bou Hedma National Park in central Tunisia. To identify the soil-water relations, the soil-water retention curve and the hydraulic conductivity of the upper soil layer (0-10 cm) were determined. Two habitats were distinguished: canopy and interspace, respectively underneath and outside the canopy of Acacia trees. The field saturated hydraulic conductivity (K fs) was higher underneath compared to outside the canopy (8.03 vs. 5.14 x 10-6 m s-1). This could be related to the shift in the wet range of the soil-water retention curve, suggesting K fs was driven by greater macroporosity. By improving the soil hydraulic properties, Acacia trees have a strong influence on the water availability within the nursed ecosystem. Therefore, scattered trees can play a central role in the restoration of degraded drylands. Differences in hydraulic properties underneath and outside the canopy underline the importance of incorporating the spatial variability when developing hydrological models on a field scale.
Soil and Tillage Research, 2009
Journal of Hydrology, 2006
There is a need to understand and analyze rainfall variability, since it is most variable in time... more There is a need to understand and analyze rainfall variability, since it is most variable in time and space, for assessing the erosive potential of rainfall and its impacts on soil erosion and conservation measures. In order to have a spatially realistic surface of the modified fournier index (MFI) and a statistically valid method, two different procedures of calculating the MFI were performed using the daily rainfall amounts recorded for 29 years in the region of Southeastern Anatolia Project (GAP), Turkey and efficiently integrating elevation and GIS. The method that calculated the MFI surface from the monthly rainfall amounts of each individual year and averaged over a number of years ðMFI j Þ was compared with the method that calculated the MFI surface from the averages of ith monthly rainfall amounts and averaged over a number of years ðMFIÞ. Results indicated that the MFI led to the lower-risk MFI classes than the MFI j. This was attributed to the fact that the MFI was statistically unable to account for the year-to-year variability in the rainfall data. An analysis with the relationship between the MFI and the coefficient of variation (CV) also suggested the total variability in the data set be better represented to have dependable MFI surfaces because of integration of elevation. Further calculations for surfaces of the precipitation concentration index (PCI) similar to the calculations made for the MFI surfaces conclusively indicated that the MFI j was valuable in determining the potential of the rains for causing soil erosion by providing information on a long-term total variability in the rainfall amount received.
Hydrology and Earth System Sciences, 2009
In many arid countries, runoff water-harvesting systems support the livelihood of the rural popul... more In many arid countries, runoff water-harvesting systems support the livelihood of the rural population. Little is known, however, about the effect of these systems on the water balance components of arid watersheds. The objective of this study was to adapt and evaluate the GIS-based watershed model SWAT (Soil Water Assessment Tool) for simulating the main hydrologic processes in arid environments. The model was applied to the 270-km 2 watershed of wadi Koutine in southeast Tunisia, which receives about 200 mm annual rain. The main adjustment for adapting the model to this dry Mediterranean environment was the inclusion of waterharvesting systems, which capture and use surface runoff for crop production in upstream subbasins, and a modification of the crop growth processes. The adjusted version of the model was named SWAT-WH. Model evaluation was performed based on 38 runoff events recorded at the Koutine station between 1973 and 1985. The model predicted that the average annual watershed rainfall of the 12-year evaluation period (209 mm) was split into ET (72%), groundwater recharge (22%) and outflow (6%). The evaluation coefficients for calibration and validation were, respectively, R 2 (coefficient of determination) 0.77 and 0.44; E (Nash-Sutcliffe coefficient) 0.73 and 0.43; and MAE (Mean Absolute Error) 2.6 mm and 3.0 mm, indicating that the model could reproduce the observed events reasonably well. However, the runoff record was dominated by two extreme events, which had a strong effect on the evaluation criteria. Discrepancies remained mainly due to uncertainties in the ob
Hydrology and Earth System Sciences, 2009
In arid and semi-arid zones, runoff harvesting techniques are often applied to increase the water... more In arid and semi-arid zones, runoff harvesting techniques are often applied to increase the water retention and infiltration on steep slopes. Additionally, they act as an erosion control measure to reduce land degradation hazards. Nevertheless, few efforts were observed to quantify the water harvesting processes of these techniques and to evaluate their efficiency. In this study, a combination of detailed field measurements and modelling with the HYDRUS-2D software package was used to visualize the effect of an infiltration trench on the soil water content of a bare slope in northern Chile. Rainfall simulations were combined with high spatial and temporal resolution water content monitoring in order to construct a useful dataset for inverse modelling purposes. Initial estimates of model parameters were provided by detailed infiltration and soil water retention measurements. Four different measurement techniques were used to determine the saturated hydraulic conductivity (K sat) independently. The tension infiltrometer measurements proved a good estimator of the K sat value and a proxy for those measured under simulated rainfall, whereas the pressure and constant head well infiltrometer measurements showed larger variability. Six different parameter optimization functions were tested as a combination of soil-water content, water retention and cumulative infiltration data. Infiltration data alone proved insufficient to obtain high model accuracy, due to large scatter on the data set, and water content data were needed to obtain optimized effective parameter sets with small confidence intervals. Correlation between the observed soil water content and the simulated values was as high as R 2 =0.93 for ten selected observation points used in the model calibration phase, with overall correlation for
Geomorphology, 2012
Wind erosion is a global environmental problem. Re-vegetating land is a commonly used method to r... more Wind erosion is a global environmental problem. Re-vegetating land is a commonly used method to reduce the negative effects of wind erosion. However, there is limited knowledge on the effect of vegetation pattern on wind-blown mass transport. The objective of this study was to investigate the effect of vegetation pattern on this phenomenon within a land unit and at the border between land units. Wind tunnel experiments were conducted with artificial shrubs representing Atriplex halimus. Wind runs at a speed of 11 m s-1 were conducted and sand translocation was measured after 200-230 s using a graph paper prepared for this purpose. This research showed that: 1) the transport within a land unit is affected by the neighboring land units and by the vegetation pattern within both the unit itself and the neighboring land units; 2) re-vegetation plans for degraded land can take into account the 'streets' effect (zones of erosion areas similar to streets); 3) the effect of neighboring land units includes sheltering effect and the regulation of sediment passing from one land unit to the neighboring land units and 4) in addition to investigation of the general effect of vegetation pattern on erosion and deposition within the region, it is important to investigate the redistribution of sediment at smaller scales depending on the scope of the project.
CATENA, 2008
Soil erosion from agricultural lands is a serious problem on the Chinese Loess Plateau. In total,... more Soil erosion from agricultural lands is a serious problem on the Chinese Loess Plateau. In total, 28 field rainfall simulations were carried on loamy soils under different management practices, namely conventional tillage (CT), no till with mulch (NTM), reduced tillage (RT), subsoiling with mulch (SSM), subsoiling without mulch (SS), and two crops per year (TC), to investigate (i) the effects of different soil management practices on runoff sediment and (ii) the temporal change of runoff discharge rate and sediment concentration under different initial soil moisture conditions (i.e. initially dry soil surface, and wet surface) and rainfall intensity (85 and 170 mm h − 1) in the Chinese Loess Plateau. NTM was the best alternative in terms of soil erosion control. SSM reduced soil loss by more than 85% in 2002 compared to CT, and its effects on runoff reduction became more pronounced after 4 years consecutive implementation. SS also reduced considerably the runoff and soil loss, but not as pronounced as SSM. TC resulted in a significant runoff reduction (more than 92%) compared to CT in the initial 'dry' soil, but this effect was strongly reduced in the initial 'wet' soil. Temporal change of runoff discharge rate and sediment concentration showed a large variation between the different treatments. In conclusion, NTM is the most favorable tillage practices in terms of soil and water conservation in the Chinese Loess Plateau. SSM can be regarded as a promising measure to improve soil and water conservation considering its beneficial effect on winter wheat yield.
Biology and Fertility of Soils, 2008
Laboratory incubation experiments were conducted to study the C and N mineralization dynamics of ... more Laboratory incubation experiments were conducted to study the C and N mineralization dynamics of crop residues (fine roots and straw) of the two main crops (winter wheat and peanut) in the Chinese Loess Plateau under different ways of incorporation. The C mineralization patterns of the soil amended with winter wheat residues differed greatly, and the highest C mineralization was observed in the treatment with winter wheat straw incorporated (39% of the total added C mineralized). The way of straw placement had only a minor effect on the pattern of C mineralization for peanut. Generally, winter wheat residues showed a stronger immobilization than peanut residues during the incubation period, without any net N release. Winter wheat straw incorporated showed the strongest N immobilization with 35 mg kg −1 (equivalent to 27% of added N) immobilized at the eighth week. This study indicated that retaining crop residues at the soil surface in the dry land soils of the Chinese Loess Plateau is beneficial for C sequestration. It also showed that N immobilization occurs only during a limited period of time, sufficient to prevent part of the mineral N pool from leaching, and that net N mineralization can be expected during the subsequent cropping season, thus enhancing synchronization of N supply and demand.
… Methods in Water …, 2010
In arid and semi-arid zones runoff harvesting techniques are often applied to increase the water ... more In arid and semi-arid zones runoff harvesting techniques are often applied to increase the water retention and infiltration on steep slopes. Additionally, they act as an erosion control measure to reduce land degradation hazards. Both in literature and in the field, a large variety of runoff collecting systems are found, but a rigorous evaluation of their efficiency is lacking. Therefore, detailed measurements were performed on a semi-arid slope in central Chile to allow identification of the effect of a simple water harvesting ...
Sociedade & Natureza
Evaluating runoff and field soil losses by either empirical or physically based erosion models, r... more Evaluating runoff and field soil losses by either empirical or physically based erosion models, requires an understanding and assessment of rainfall characteristics affecting the erosion (sub)processes. In spite of shortcomings and many criticisms on the worldwide use and applicability of RUSLE (Revised Soil Loss Equation), this empirical model still gains worldwide popularity for estimating erosion on a farmfield or micro-catchment scale. One of the limits of the model is the assessment of its climate factor in terms of rain energy and maximum storm intensity, both factors combined in the erosivity factor R, although they not easily available nor easily determinable. Therefore attempts are made to look for more easily determinable rain characteristics such as the monthly precipitation for evaluating the rain aggressiveness (or rain erosivity) on a monthly or yearly basis. As agro-climatological zones have typical rain distributions and rain concentrations, the Modified Fournier Ind...
ABSTRACT The distribution and aggressiveness of the precipitation in Los Andes region of Venezuel... more ABSTRACT The distribution and aggressiveness of the precipitation in Los Andes region of Venezuela was evaluated by analyzing monthly precipitation data from 56 meteorological stations. The modified Fournier index (FMI) and precipitation concentration index (PCI) were calculated to assess rain aggressiveness and rain distribution, respectively. The index values were analyzed using univariate and geostatistical analysis adjusting semivariograms to theoretical models. For mapping of the spatial distribution of indices punctual kriging interpolation method was carried out, generating the appropriate file, which was finally edited in ArcView. The results pointed out that the region under study has high and very high rainfall aggressiveness over 70% of the entire territory, while the distribution of rainfall was moderately seasonal. Rainfall aggressiveness, in combination with the predominance of a sloped relief increases the risks of soil degradation, with the annual rainfall ranging from 500 to 3900 mm. In this sense, soil conservation practices should be considered on existing crop production systems in the evaluated region.
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Papers by Donald Gabriels