Dambo Farming In Zimbabwe: Water Management, Cropping and Soil Potentials for Smallholder Farming in the Wetlands

A conference paper on water management in Africa.Rising population pressures in Africa have caused the agricultural use of wetlands to become increasingly important. Developing large surface irrigation infrastructures, as Asia did during the sixties and seventies, often proves to be too costly for Africa. This makes wetlands, with their relatively good water availability and high soil fertility, an interesting alternative for increasing food production. Wetland use offers economic advantages as well. Farming on wetlands is a labor-intensive process, while surface irrigation is capital- intensive

Changes in land cover, rainfall and stream flow in Upper Gilgel Abbay catchment, Blue Nile basin – Ethiopia

In this study we evaluated changes in land cover and rainfall in the Upper Gilgel Abbay catchment in the Upper Blue Nile basin and how changes affected stream flow in terms of annual flow, high flows and low flows. Land cover change assessment was through classification analysis of remote sensing based land cover data while assessments on rainfall and stream flow data are by statistical analysis. Results of the supervised land cover classification analysis indicated that 50.9 % and 16.7 % of the catchment area was covered by forest in 1973 and 2001, respectively. This significant decrease in forest cover is mainly due to expansion of agricultural land. \ud \ud By use of a change detection procedure, three periods were identified for which changes in rainfall and stream flow were analyzed. Rainfall was analyzed at monthly base by use of the Mann-Kendall test statistic and results indicated a statistically significant, decreasing trend for most months of the year. However, for the wet season months of June, July and August rainfall has increased. In the period 1973–2005, the annual flow of the catchment decreased by 12.1 %. Low flow and high flow at daily base were analyzed by a low flow and a high flow index that is based on a 95 % and 5 % exceedance probability. Results of the low flow index indicated decreases of 18.1 % and 66.6 % for the periods 1982–2000 and 2001–2005 respectively. Results of high flows indicated an increase of 7.6 % and 46.6 % for the same periods. In this study it is concluded that over the period 1973–2005 stream flow has changed in the Gilgel Abbay catchment by changes in land cover and changes in rainfall

Hoe snelle stroming door preferente banen het grondwater kan verontreinigen

Pesticiden en andere chemicaliën blijken vaak sneller in het grondwater terecht te komen dan modellen voorspellen. De meeste van deze modellen zijn namelijk gebaseerd op de veronderstelling dat de bodem homogeen is, het water in de onverzadigde zone verticaal infiltreert en het vochtfront evenwijdig is aan het bodemoppervlak. In werkelijkheid stroomt het water vaak via preferente stroombanen door de bodem. Deze stroombanen kunnen in nagenoeg alle gronden optreden. Water en daarin opgeloste stoffen bereiken hierdoor veel sneller het grondwater dan op grond van de gemiddelde verblijftijd verwacht zou worden. Vooral ondiep grondwater en oppervlaktewater hebben door dit proces een vergrote kans op verontreiniging

Unsupervised classification of saturated areas using a time series of remotely sensed images

The spatial distribution of saturated areas is an important consideration in numerous applications, such as water resource planning or siting of management practices. However, in humid well vegetated climates where runoff is produced by saturation excess processes on hydrologically active areas (HAA) the delineation of these areas can be difficult and time consuming. A technique that can simply and reliably predict these areas would be a powerful tool for scientists and watershed managers tasked with implementing practices to improve water quality. Remotely sensed data is a source of spatial information and could be used to identify HAAs. This study describes a methodology to determine the spatial variability of saturated areas using a temporal sequence of remotely sensed images. The Normalized Difference Water Index (NDWI) was derived from medium resolution Landsat 7 ETM+ imagery collected over seven months in the Town Brook watershed in the Catskill Mountains of New York State and used to characterize the areas susceptible to saturation. We found that within a single land cover, saturated areas were characterized by the soil surface water content when the vegetation was dormant and leaf water content of the vegetation during the growing season. The resulting HAA map agreed well with both observed and spatially distributed computer simulated saturated areas (accuracies from 49 to 79). This methodology shows that remote sensing can be used to capture temporal variations in vegetation phenology as well as spatial/temporal variation in surface water content, and appears promising for delineating saturated areas in the landscape

Spatiotemporal variability of hydrologic soil properties and the implications for overland flow and land management in a peri-urban Mediterranean catchment

Planning of semi-urban developments is often hindered by a lack of knowledge on how changes in landuse affect catchment hydrological response. The temporal and spatial patterns of overland flow source areas and their connectivity in the landscape, particularly in a seasonal climate, remain comparatively poorly understood. This study investigates seasonal variations in factors influencing runoff response to rainfall in a peri-urban catchment in Portugal characterized by a mosaic of landscape units and a humid Mediterranean climate. Variations in surface soil moisture, hydrophobicity and infiltration capacity were measured in six different landscape units (defined by land-use on either sandstone or limestone) in nine monitoring campaigns at key times over a one-year period. Spatiotemporal patterns in overland flow mechanisms were found. Infiltration-excess overland flow was generated in rainfalls during the dry summer season in woodland on both sandstone and limestone and on agricultural soils on limestone due probably in large part to soil hydrophobicity. In wet periods, saturation overland flow occurred on urban and agricultural soils located in valley bottoms and on shallow soils upslope. Topography, water table rise and soil depth determined the location and extent of saturated areas. Overland flow generated in upslope source areas potentially can infiltrate in other landscape units downslope where infiltration capacity exceeds rainfall intensity. Hydrophilic urban and agricultural-sandstone soils were characterized by increased infiltration capacity during dry periods, while forest soils provided potential sinks for overland flow when hydrophilic in the winter wet season. Identifying the spatial and temporal variability of overland flow sources and sinks is an important step in understanding and modeling flow connectivity and catchment hydrologic response. Such information is important for land managers in order to improve urban planning to minimize flood risk

Effect of surfactant on fingered flow in laboratory golf greens

Preferential flow in golf greens causes turfgrass quality deterioration and shortens solute travel time. Little is known how surfactants affect the flow pattern in golf greens. This study visualizes the changes in flow patterns and compares leaching of a nonadsorbed and an adsorbed chemical as a result of the addition of a nonionic surfactant in a simulated United States Golf Association (USGA) putting green profile in a two-dimensional sand slab chamber. The USGA putting greens have three distinct layers that become coarser with depth. The addition of the surfactant changed the unstable preferential flow pattern to a uniform flat wetting front in the first 11 cm. Below this, small perturbations occurred, and at a depth of 17 cm, the wetting front became unstable and formed six fingers. After the first 17 cm, the infiltration process was not affected by the addition of the surfactant. Water infiltrated into the second layer as a wavy front and developed a single finger at the third layer. After the irrigation was stopped and water reapplied, the water followed the same pathways established during the first infiltration cycle. Chloride moved two times more slowly in the surfactant-treated profile compared with the nontreated profile. A second application of the surfactant at the same rate did not alter the flow patterns. The study showed that surfactants can provide an inexpensive way to decrease the travel velocity of the pollutants and increase the efficacy of water and applied chemicals through coarse-textured turfgrass profiles

Effect of surfactant on fingered flow in laboratory golf greens

Preferential flow in golf greens causes turfgrass quality deterioration and shortens solute travel time. Little is known how surfactants affect the flow pattern in golf greens. This study visualizes the changes in flow patterns and compares leaching of a nonadsorbed and an adsorbed chemical as a result of the addition of a nonionic surfactant in a simulated United States Golf Association (USGA) putting green profile in a two-dimensional sand slab chamber. The USGA putting greens have three distinct layers that become coarser with depth. The addition of the surfactant changed the unstable preferential flow pattern to a uniform flat wetting front in the first 11 cm. Below this, small perturbations occurred, and at a depth of 17 cm, the wetting front became unstable and formed six fingers. After the first 17 cm, the infiltration process was not affected by the addition of the surfactant. Water infiltrated into the second layer as a wavy front and developed a single finger at the third layer. After the irrigation was stopped and water reapplied, the water followed the same pathways established during the first infiltration cycle. Chloride moved two times more slowly in the surfactant-treated profile compared with the nontreated profile. A second application of the surfactant at the same rate did not alter the flow patterns. The study showed that surfactants can provide an inexpensive way to decrease the travel velocity of the pollutants and increase the efficacy of water and applied chemicals through coarse-textured turfgrass profiles