Maarten A. Siebel

Assessment of the major water and nutrient flows in the Chivero catchment area, Zimbabwe

Abstract The management of water resources is best done on an integrated catchment basis; taking into account the impact of pollution on available water quantity and quality. A study was done from June 2000 to March 2001 in Harare, Zimbabwe, to establish major water and nutrient flows in the Chivero subcatchment area. Lake Chivero is part of the major water supply source for Harare and is located downstream. Discharges from sewage treatment works, urban and rural agriculture, and industries, have caused a severe stress on water quality of the eutrophic lake. The study quantified major water flows through gauging stations on rivers, current metering, and hydrological modelling. Flows for raw water abstractions, sewage and water treatment works were obtained from continuous metering and pumping readings. Water samples were collected monthly for sewage, rivers and Lake Chivero, and analysed according to standard methods. Water and nutrient balances were developed. The results showed that urban water demand would exceed available treatment capacity by the year 2003. Sewage effluent presently is the major source of nutrients in the rivers. The absence of adequate flushing of the lake gives rise to accumulation of the nutrients in the lake. It was concluded that the current situation is not sustainable. It was recommended that the next water supply source be found outside the catchment so that its water can help flush the lake, and that sewage be treated to higher standards (without any pasture irrigation) and discharged directly into the rivers for recycling. The other alternative is to recycle the nutrients in controlled urban agriculture, thereby reducing fertiliser runoff. Stricter regulations and regular monitoring was recommended to control industrial pollution.

Soil Moisture Dynamics Modeling Considering the Root Compensation Mechanism for Water Uptake by Plants

A numerical model is developed in this study for simulating soil moisture flow in layered soil profile with plant growth. A dynamic root compensation mechanism (RCM) is used for a nonuniform root distribution pattern to compute water uptake by plants in a moisture scarce environment. The governing soil moisture flow equation integrated with the roots water uptake function is solved numerically by the implicit finite difference method coupled with the Picard iteration technique. The model is first tested for a barren layered soil profile using numerical simulation data available in the literature. A nonlinear function for water uptake by roots is then incorporated in the flow equation and the rate of water removal is simulated with and without considering the RCM for a characteristic example under optimal and water scare conditions. The model is finally applied to a rain-fed wheat (Triticum aestivum) field using a dynamic root growth model. The simulation considering the RCM shows better agreement with the...

Soil moisture flow modeling with water uptake by plants (wheat) under varying soil and moisture conditions.

A variably saturated soil moisture flow model is developed for planted soils with depth varying properties by incorporating a nonuniform macroscopic root water uptake function. The model includes spatial and temporal variation of the root density with dynamic root growth for simulating water uptake by plants along with the impact of soil moisture availability. The governing partial differential moisture flow equation integrated over the depth with a plant water uptake term is solved numerically by the implicit finite difference method using an iterative scheme. The model is first tested for barren soils for two profiles considering constant and depth varying soil characteristics under constant inflow condition. The results obtained are later tested with experimental data available in the literature. A nonuniform plant water uptake term is subsequently incorporated in the model and water uptake by wheat plants under different soil moisture availability conditions is studied. Finally, the moisture flow model is validated with field data of rain fed wheat Triticum aestivum using a dynamic root growth model for a layered root zone soil profile. The simulated soil moisture regime of the layered root zone shows a reasonably good agreement with the observed data.

A strategic framework for managing wastewater : A case study of Harare, Zimbabwe

Abstract Water quality is an urgent problem in Harares main water supply source, Lake Chivero, while water scarcity will be a problem soon. This study focuses on sustainable ways of urban water and nutrient (nitrogen and phosphorus) management in Harare, part of the Lake Chivero catchment. The Dublin Principles on water conservation, environmental protection, reuse, and economic utilization of water were used as a basis for formulating innovative and responsive solutions. External and internal drivers for future evolution of wastewater approaches are identified. Water conservation, treatment, and reuse strategies are developed including water-saving devices, regulation, leak detection and repair, and including wastewater treatment and reuse. The three levels where wastewater can be handled—onsite, decentralized, and centralized—are discussed and solutions formulated. Based on this, a strategic framework, the so-called “3-Step Strategic Approach,” is then developed aimed at the sustainable use of water, nutrients, and other resources in the urban environment. This 3-Step Strategic Approach consists of pollution prevention and minimization (Step 1), reuse after treatment (Step 2), and discharge into the environment with stimulation of self-purification capacity of the receiving environment (Step 3). The 3-Step Strategic Approach was subsequently applied to the Lake Chivero catchment, which demonstrated that these strategies could substantially reduce current water pollution and future water scarcity problems in Harare.