Pagsanjan-Lumban catchment, Philippines: Summary of biophysical characteristics of the catchment, background to site selection and instrumentation

The purpose of this paper is to provide background information and sample locations for research on agricultural water management issues of the lake Laguna de Bay, Philippines, addressed in this special issue. We introduce the key features of the lake, the rationale for catchment selection, agricultural land uses of the selected catchment, and describe the instrumentation installed for water quality measurements. Laguna de Bay is the largest freshwater lake in the Philippines and the second largest in Southeast Asia. It has a surface area of 900km2 in the form of three bays. The eastern bay of the lake is important for aquaculture and leisure activities. It is also planned as a future source of drinking water for Metro Manila as it is the least impacted by human activity of the three bays. In contrast to the central and western bays, the eastern bay has little urbanisation or industrial activities in its catchments. The main risk of water quality degradation is from diffuse non-point source pollution from its predominantly agricultural catchments. There are 24 catchments surrounding the lake of which the Pagsanjan-Lumban catchment is the second biggest. However, the Pagsanjan-Lumban catchment provides the largest flow of water into the lake. The predominant land uses in this catchment are coconut plantations (33%), and cropland mixed with coconut plantation (26%). Rice production is also an important land use in the catchment. For this study the Pagsanjan-Lumban catchment was instrumented with automatic water samplers and water level loggers at 4 sites that are representative of the major sources of agricultural contaminants in the catchments surrounding the eastern bay of the lake. This allowed the quantification over 2 years of concentrations and loads of sediment, nutrients and selected pesticides being transported off-site at the sampling sites. Background information and details of the instrumentation of this catchment are given in this manuscript and details of the findings from the water analyses and modelling are given in subsequent manuscripts.