The University of San Carlos-Water Resources Center (USC-WRC) has been doing field measurements on rainfall, water levels in rivers, ground water level, stream flow, and evaporation. The center installed rain gauges, staff gauges, and meteorological stations all over the island of Cebu. Most of their field instruments are still operational and well maintained. The center has compiled lots of data and digitized maps for several parts of the Visayas. Furthermore, the center has the expertise in the field of hydrology and water resources assessment. For the cited capability, the USC Phil LiDAR 1 Team has tapped the consultancy services of the Hydrology Section of USC-WRC in the initial stages of the project.
Figure 2.1. The Mananga Watershed is located in the central part of the Province of Cebu.
On May 7, 2014, Saturday the USC Phil LiDAR 1 Team conducted an ocular inspection in the Mananga Watershed to familiarize the place. In Mananga River staff gauges, which are markings that indicate water level, installed by USC-WRC were found. The staff gauge shown in Figure 2.2 is marked on the column of the bridge at Camp 4, Talisay City. The highest mark is 3 m.
Figure 2.2. The markings on the column of the bridge (staff gauges) indicate the water level from the left bank of the river.
One concern that can be observed from Figure 2.2 is the lowering of the river bed which is manifested by the emergence of a portion of the pile cap. The lowering of the river bed poses a threat to the structural integrity of the connection between the piles and the pile cap because they will be exposed to the impact of strong water current during flood events. In addition, the lowering of the river bed has implication on the water storage capacity of the watershed.
About 300 m downstream of the staff gauges described in Figure 2.2, one can find another set of staff gauges installed at different elevations (see Figure 2.3) by USC-WRC. These staff gauges were put in place so that low and high water levels can be recorded. They were securely fastened to withstand the impact of the water current. During high flows both staff gauges can be read simultaneously to get an idea of the water surface slope, which is an important parameter in estimating the flow velocity.
A number of rain gauges were also installed in the Mananga Watershed. Two prototypes of these rain gauges are shown in Figure 2.4. The white one is a manual rain gauge, wherein, the rainfall collected by the instrument is stored in the lower cylinder and this will be measured by an observer using a graduated cylinder. The measurement is done twice a day one at 8:00 AM and the other at 5:00 PM. This type of rain gauge can only provide average daily intensity. Actual rainfall duration and intensities at shorter time intervals cannot be extracted. The black one is a recording tipping bucket type rain gauge. The red box fastened at the post supporting the instrument is a data logger that stores the rainfall collected by the rain gauge.
The data can be downloaded to a laptop computer. The recording rain gauge can provide rainfall intensities at shorter intervals; say 5, 10, or 15 minutes and it can also provide the duration of the rainfall event. The two rain gauges are placed side by side for checking purposes or as backup if in case the recording rain gauge fails to record the rain. The two gauges are installed in Jaclupan, Talisay City particularly at the site where the siltation and infiltration basins for the water impounded by the 7-m high Jaclupan weir. The facilities are maintained by the Metro Cebu Water District (MCWD). There are a number of rain gauges installed by USC-WRC all over Metro Cebu for MCWD projects and of these about 7 or 8 rain gauges can be made as reference for watershed study of Mananga.
A Stevenson screen (see Figure 2.5) was also installed in Mananga by USC-WRC when the center undertook a project for MCWD in 2011. A Stevenson screen is a louvered enclosure which is made of wood and painted with white color. It is used to house meteorological instruments such as thermometers and hygrometers. The design of such enclosure is to allow circulation of air while protecting the instruments from direct rays of the sun and direct exposure to rain. The enclosure shown in Figure 2.5 houses a thermometer and a hygrometer that continuously monitor temperature and humidity, respectively. The data collection started on July 10, 2011 and it is continuing up to the present. The location of this meteorological station is at the infiltration basin of the Jaclupan weir.