USING THE PLANNING AND MANAGEMENT MODEL OF LAKES AND RESERVOIRS

Abstract

The future condition of a lake ecosystem can be predicted through the interpretation of past and present responses to change. The objective of this study was to predict the changes in the eutrophic status of Lake Chivero using the Planning and Management Model of Lakes and Reservoirs (PAMOLARE) as a tool in planning its rehabilitation. The model estimated the effect of nutrient reduction through three management scenarios, which are the use of natural wetlands and use of efficient wastewater treatment systems against the existing management system. Modelling parameters were gathered through, experimental work, literature review and information from responsible authorities. The current eutrophic status of Lake Chivero was evaluated by analysing different physico-chemical variables from the lake’s main and minor tributaries. Water samples were collected from eight sites, Marimba, Mukuvisi, Manyame, Kuwadzana, Budiriro, Glenview (rivers), lake midpoint and lake near wall. Sampling was conducted from the 29th of October 2010 to the 31st of January 2011. Physical parameters, dissolved oxygen (DO), turbidity, conductivity, total dissolved solids (TDS), pH and temperature were measured on site. Chemical analyses on composite samples were done for total nitrogen (TN), total phosphorus (TP) and chlorophyll-a using standard methods. Concentrations were determined by colorimetric methods, using a spectrophotometer. TN and TP concentrations were multiplied by river flow discharge over area, translating into nutrient loads. The results indicated that Lake Chivero is hypereutrophic, with mean phosphorus concentration of 2.77 mgL-1 and nitrogen 3.21 mgL-1. The quality of the water varied within time and space, which was greatly influenced by the intensity and type of land use activities. Secchi depth was lowest from the rivers rarely exceeding 0.5 m and highest from the lake points reaching 1.9 m. The rivers recorded the least oxygen levels with Budiriro and Mukuvisi recording as low as 0.0 mgL-1. Higher conductivity values were recorded from the rivers as compared to the lake points. Conductivity and TDS showed no relationship with the depth profile. DO, pH and temperature from the lake sites decreased with depth with a poor gradient. Mixing was experienced on some sampling days shown by absence of the thermocline. The multiple analysis of variance (MANOVA) for the physico-chemical parameters indicated that temperature, % oxygen saturation, DO, pH, salinity, TN and chlorophyll-a had some significant difference (p≤0.05) with site. Conductivity, ORP and TDS had significant difference (p≤0.05) to be about 493 tonnes per annum. The non-point phosphorus source competed closely with 634 tonnes per annum phosphorus from point sources. The estimated wetland area in the catchment has capacity to remove about 80 000 tonnes of phosphorus per annum and about 9.97*104 tonnes of nitrogen per annum. Phosphorus in water would decrease from 2.77 to 0.22 mgL-1 in the next 6.5 years. Nitrogen levels in water will also decrease from 3.16 to 3.06 mgL-1 in the next 4 years. Results of the model scenario runs reveal that an 82% reduction of the current loadings would improve the water quality from hyper-eutrophic to eutrophic conditions. The modelling effort has yielded valuable information that can be used by decision makers for the evaluation of different management strategies on reducing point and non-point nutrient loadings into Lake Chivero.