Scope effects in contingent valuation: an application to the valuation of irrigation water quality improvements in Infulene Valley, Mozambique

This study uses the double-bounded bid elicitation format to test whether the willingness to pay (WTP) of 244 randomly selected residents of Maputo and Matola cities for wastewater quality improvements in the Infulene Valley is sensitive to internal and external scope. The Infulene Valley was selected because its wastewater is used as an input in vegetable irrigation. WTP was elicited and compared when the level of wastewater treatment was 100 % and when it was 50 %. The results show that the majority of those interviewed display high levels of knowledge regarding the risks associated with poor quality irrigation water, and that they have attitudes and perceptions receptive to a policy that aims to improve irrigation water quality. The WTP responses passed the bottom up (t= 15.28, p=0.000) and top down (t=14.07, p=0.000) internal and external (t=13.43, p=0.000) scope tests, suggesting that the level of wastewater treatment significantly influences households' WTP. The following variables were statistically significant in the WTP model: income, age, education level, household size, gender, whether the household considers water scarcity as a priority issue, knowledge of the unsuitability of Infulene Valley water for vegetable irrigation, and whether the household is aware that the Infulene Valley is an important supplier of fresh vegetables to Maputo and Matola residents. The study concluded that the level of water treatment (high quality of treated wastewater) is a significant factor of preference over the alternative policy in wastewater treatment. The following recommendations derive from the study: policy makers should consider wastewater treatment planning and they should develop an irrigation water pricing system, as well as conservation practices to manage pollution problems at Infulene Valley. While this study provides an estimate of household values for irrigation water quality improvements in the Infulene Valley, is ultimately up to policy makers at the city and country levels to implement any changes.