Title: Difference of water policy efficacy across South African water management areas
Thierry Yerema COULIBALY1*, Mihoko Wakamatsu1, Moinul Islam, Fukai Hiroki, Shunsuke Managi1
１．Introduction
The legacy of inequitable water policy under apartheid continues to impact water services in economically lessdeveloped and rural areas in South Africa. Previous studies typically examine this relationship either by using datacollected at a national level for large-scale research or by using aggregated data collected at the provincial or
municipal level for a specific locality only. This study attempts to perform a nationwide analysis using fine-scale(water management level) data to give a spatial representation of the efficacy of water policies in South Africa.
National level or province-level research cannot be used to tackle economic and/or the legacies of racial injustices,as within each province there is considerable diversity across economic prosperity and areas related to former
apartheid colonization.
The analysis is based on the assumption that economic development may degrade water quality through somewater polluting land use and it may improve water policies and therefore improve water quality. Since water
quaternary management areas are the smallest unit of policy implementation, a positive relationship indicateseffective water quality regulation policies and/or legacies of the effectiveness of water policies implemented
during the apartheid era, while a negative correlation may indicate poor or ineffective policy. Hence, by controlling
for land use activities appearing as main pollution causes, we attempt to evaluate the efficacy of water policies
which can be explained by economic development.
２．Methodology
With a sample collected by the Water Resources Commission in South Africa, we use the natural logarithm ofTotal Dissolved Solids (TDS), Sulfate, Nitrogen, and phosphorus concentrations in main surface water as waterpollutant indices. We use the natural logarithm of satellite night-time light digital number as economic
development proxy. According to the literature, they are the finest proxy variables for our study unit. Finally, thesurface of land use, population density, runoff, and the total area of the water quaternary were used as control
variables.
To investigate the impact of economic development on water pollutants, we used two spatial econometric models.
First, a Geographically Weighted Regression (GWR) was applied to estimate the spatially varying relationshipbetween water quality and economic development in the country. It allows to graphically detect the water
management areas in which there might be a positive correlation between economic development and waterpollution.
Second, a spatial Durbin Model (SDM) was performed to test the significance of the results of the GWR. In theSDM, we assumed a correlation of the water pollutants with neighboring observation described by a contiguitymatrix. Concerning the independent variables, we assume a correlation of economic development between thedifferent areas characterized by a Euclidean distance matrix. The land use variables are estimated by an inverse1

Department of Urban Engineering, School of Engineering, Kyushu University* 〒819-0395, 744 Motooka, Nishi-ku, Fukuoka, TEL : 080-3993-1993 Email: yerema.coul@gmail.com

distance-weighted metrics of the land use relative to the nearest water bodies in each region. This metric allows
land uses closer to water bodies to have a stronger impact on water quality.
３．Results
The predictions of the GWR approximately show that the relationship between economic development and waterpollutants is negative in the West of the country and positive in the East. This pattern could be the results of a
substantial difference in human activities (land use) between the two parts of the country. Alternatively, it could
be related to the former separation of land during the apartheid era as black dominated regions were roughly
concentrated in the eastern part of the country.
We then test for the significance of these results and their possible causes with the SDM. The results of the SDM
highlights that mining, agricultural, and industrial activities are significantly related to water pollution. However,even after controlling for the effect of land use, the relationship between economic development and water quality
in the Eastern part of the country is significantly greater than in the west.
Subsequently, by matching a racial concentration map in 1970 with the water management areas (unit of thisstudy), we investigate the hypothesis that the former black dominated areas could have the lowest surface waterquality. However, the results rejected this hypothesis.

Figure 1 The GWR of water quality on economic development while controlling for the effect of runoff
４．Concluding remarks
Our findings highlight a significantly stronger positive relationship between water quality and economic
development in the East of the country. Then, after controlling for types of pollutant-causing land use, we suggestthat the variation in this impact of economic development can be found in the structure of the water management
institutions in South Africa. Upon further investigation, we deduce that the causal relationship varies at the levelof local and provincial water management institutions.