Amy S. Collick

Applicability of models to predict phosphorus losses in drained fields: a review.

Most phosphorus (P) modeling studies of water quality have focused on surface runoff loses. However, a growing number of experimental studies have shown that P losses can occur in drainage water from artificially drained fields. In this review, we assess the applicability of nine models to predict this type of P loss. A model of P movement in artificially drained systems will likely need to account for the partitioning of water and P into runoff, macropore flow, and matrix flow. Within the soil profile, sorption and desorption of dissolved P and filtering of particulate P will be important. Eight models are reviewed (ADAPT, APEX, DRAINMOD, HSPF, HYDRUS, ICECREAMDB, PLEASE, and SWAT) along with P Indexes. Few of the models are designed to address P loss in drainage waters. Although the SWAT model has been used extensively for modeling P loss in runoff and includes tile drain flow, P losses are not simulated in tile drain flow. ADAPT, HSPF, and most P Indexes do not simulate flow to tiles or drains. DRAINMOD simulates drains but does not simulate P. The ICECREAMDB model from Sweden is an exception in that it is designed specifically for P losses in drainage water. This model seems to be a promising, parsimonious approach in simulating critical processes, but it needs to be tested. Field experiments using a nested, paired research design are needed to improve P models for artificially drained fields. Regardless of the model used, it is imperative that uncertainty in model predictions be assessed.

Eco-hydrological impacts of Eucalyptus in the semi humid Ethiopian Highlands: the Lake Tana Plain

Abstract Eucalyptus is the tree of choice for wood production by farmers in Ethiopia. Although there are many claims about its harmful effect on ecology and water availability, little actual research exists. The main objective of this study was, therefore, to study the extent of harm of Eucalyptus on the ecosystem. This study was conducted at the Koga Watershed near Lake Tana in Ethiopia. Twenty-five farmers were interviewed and a field experiment with three replications was carried out to quantify the effect of Eucalyptus on various soil physical and chemical properties and maize crop measurements and to compare bulk density, soil moisture contents, maize crop counts and shading effects in fields bordered by Eucalyptus and Croton macrostachyus. Our results show that Eucalyptus decreased both soil nutrients and maize yields within 20 m of the trees. Although moisture content was not affected during the monsoon, it decreased faster within 30 m of the Eucalyptus trees than elsewhere. Soils become water repellent, too. Local farmers’ perception agreed with our experimental findings and indicated that Eucalyptus trees are exhausting the once productive land. They also reported that Eucalyptus dries up springs. Despite this, the growers insist on planting Eucalyptus because of its cash income.

A Saturation Excess Erosion Model

Abstract. Scaling-up sediment transport has been problematic because most sediment loss models (e.g., the Universal Soil Loss Equation) are developed using data from small plots where runoff is generated by infiltration excess. However, in most watersheds, runoff is produced by saturation excess processes. In this article, we improve an earlier saturation excess erosion model that was only tested on a limited basis, in which runoff and erosion originated from periodically saturated and severely degraded areas, and apply it to five watersheds over a wider geographical area. The erosion model is based on a semi-distributed hydrology model that calculates saturation excess runoff, interflow, and baseflow. In the development of the erosion model, a linear relationship between sediment concentration and velocity in surface runoff is assumed. Baseflow and interflow are sediment free. Initially during the rainy season in Ethiopia, when the fields are being plowed, the sediment concentration in the river is limited by the ability of the surface runoff to move sediment. Later in the season, the sediment concentration becomes limited by the availability of sediment. To show the general applicability of the Saturation Excess Erosion Model (SEEModel), the model was tested for watersheds located 10,000 km apart, in the U.S. and in Ethiopia. In the Ethiopian highlands, we simulated the 1.1 km 2 Anjeni watershed, the 4.8 km 2 Andit Tid watershed, the 4.0 km 2 Enkulal watershed, and the 174,000 km 2 Blue Nile basin. In the Catskill Mountains in New York State, the sediment concentrations were simulated in the 493 km 2 upper Esopus Creek watershed. Discharge and sediment concentration averaged over 1 to 10 days were well simulated over the range of scales with comparable parameter sets. The Nash-Sutcliffe efficiency (NSE) values for the validation runs for the stream discharge were between 0.77 and 0.92. Sediment concentrations had NSE values ranging from 0.56 to 0.86 using only four calibrated sediment parameters together with the subsurface and surface runoff discharges calculated by the hydrology model. The model results suggest that correctly predicting both surface runoff and subsurface flow is an important step in simulating sediment concentrations.

Watershed Hydrology of the (Semi) Humid Ethiopian Highlands

Understanding the basic relationships between rainfall, runoff and soil loss is vital for effective management and utilization of water resources and soil conservation planning. A study was conducted in three small watersheds in or near the Blue Nile basin in Ethiopia, with long-term records of rainfall and discharge. To better understand the water movement within the watershed, piezometers were installed and infiltration rates were measured in the 2008 rainy season. We also reanalyzed the discharge from small plots within the watersheds. Infiltration rates were generally in excess of the rainfall rates. Based on this and plot discharge measurements, we concluded that most rainfall infiltrated into the soil, especially in the upper, steep and well-drained portions of the watershed. Direct runoff is generated either from saturated areas at the lower and less steep portions of the hill slopes or from areas of exposed bedrock. Using these principles, a simple distributed watershed hydrology model was developed. The models reproduce the daily discharge pattern reasonably well for the small watershed and the 10-day discharge values for the whole Blue Nile Basin in Ethiopia. The simplicity and scalability of the model hold promise for use in un-gauged catchments.

Spatial and Temporal Patterns of Soil Erosion in the Semi-humid Ethiopian Highlands: A Case Study of Debre Mawi Watershed

The effectiveness of water management interventions is hampered by the lack of knowledge about the spatial distribution of runoff and associated soil loss. A study was conducted in the 95-ha Debre Mawi watershed in the Upper Blue Nile basin to understand where and when runoff and erosion takes place on the landscape. During the rainy phase of the 2010 and 2011 monsoons, storm runoff and sediment concentrations were measured from five sub-watersheds. In addition, perched groundwater tables, infiltration rates, and rill erosion from agricultural fields were measured. The results show that saturation excess runoff was the main runoff mechanism because the median infiltration rate was only exceeded 3 % of the time. Early during the rainy period, runoff produced from upslope shallow soils infiltrated downslope and did not reach the outlet. At the end of July, the bottomlands became saturated, and the runoff coefficient at the outlet became greater than upslope areas. Sediment concentrations were greater in the beginning of the rainy monsoon phase when the rill network had developed on the plowed land and it becomes lowest at the end of rainy phase when rill formation stopped. At all times, the sediment concentration at the outlet was greater than upslope because both runoff losses were greater in the saturated bottomlands and loose unstructured soil was available from newly forming gullies. This research indicates that watershed management interventions to control erosion should be implemented in areas which produce the most runoff such as those shallow upland soils and bottomlands near the river that become saturated by the end of the rainy phase. In addition, for proper planning and management, runoff and erosion models should capture these dynamics.

Linking nitrogen management, seep chemistry, and stream water quality in two agricultural headwater watersheds.

Riparian seepage zones in headwater agricultural watersheds represent important sources of nitrate-nitrogen (NO-N) to surface waters, often connecting N-rich groundwater systems to streams. In this study, we examined how NO-N concentrations in seep and stream water were affected by NO-N processing along seep surface flow paths and by upslope applications of N from fertilizers and manures. The research was conducted in two headwater agricultural watersheds, FD36 (40 ha) and RS (45 ha), which are fed, in part, by a shallow fractured aquifer system possessing high (3-16 mg L) NO-N concentrations. Data from in-seep monitoring showed that NO-N concentrations generally decreased downseep (top to bottom), indicating that most seeps retained or removed a fraction of delivered NO-N (16% in FD36 and 1% in RS). Annual mean N applications in upslope fields (as determined by yearly farmer surveys) were highly correlated with seep NO-N concentrations in both watersheds (slope: 0.06; = 0.79; < 0.001). Strong positive relationships also existed between seep and stream NO-N concentrations in FD36 (slope: 1.01; = 0.79; < 0.001) and in RS (slope: 0.64; = 0.80; < 0.001), further indicating that N applications control NO-N concentrations at the watershed scale. Our findings clearly point to NO-N leaching from upslope agricultural fields as the primary driver of NO-N losses from seeps to streams in these watersheds and therefore suggest that appropriate management strategies (cover crops, limiting fall/winter nutrient applications, decision support tools) be targeted in these zones.

Monitoring State of Biomass Recovery in the Blue Nile Basin Using Image-Based Disturbance Index

The heavy dependence of the Ethiopian rural population on natural resources, particularly land, to maintain their livelihood is an underlying cause for the degradation of land and other natural resources. The Ethiopian highlands, which are the center of major agricultural and economic activities, have been eroding for many years. Various actors have undertaken reforestation programs with an aim to mitigate the land degradation problem; however, the status of these plantations has never been evaluated at a basin scale. The image-based disturbance index (DI) measures the status of the ecosystem on the basis of the ratio of long-term enhanced vegetation index (EVI) and the land surface temperature (LST). This study applied the DI to assess the current state of biomass in the upper Blue Nile basin with a focus on areas where degradation mitigation measures are implemented through reforestation campaigns. The DI maps are validated through field visits to 19 selected sites and inventory data obtained from the World Food Program (WFP) over five sites. The results showed that the largest expansion of plantations has taken place in five subbasins and is between 6 and 8.5 % of the subbasin area with expansion in the remaining 11 subbasins ranging from 3 to 5 %. Despite the very low annual rate of expansion, it can be concluded that the mitigation measures implemented through reforestation campaigns contribute to the total recovered forest area.