Important hydrologic data for watershed studies can be identified by knowing the different processes of the hydrologic cycle, which is presented in Figure 1.1. These processes are evaporation, precipitation, infiltration, overland flow generation, percolation, and seepage. To understand these processes field measurements have to be conducted. These measurements include air temperature, humidity, wind speed, rainfall, soil moisture, soil cover, slope, infiltration rate, evaporation rate, depth and velocity of current in the river, river cross sections, and ground water fluctuations. Due to the high variability of the different hydrologic processes it is preferable to have continuous data that covers long period of time to capture the variation of the processes with time. It is also preferable to have measurements in as many locations as possible to capture the variation with space.

Planning for field measurements could be very difficult if the planner has no idea of the actual topography, the kind of terrain, type of vegetation, accessibility of the river and streams, the condition of the roads, and other factors that might determine the appropriate methodology to be implemented. For this reason the first few months of the first year of implementation of the Phil LIDAR 1 Project in Central Visayas (Region 7) were spent on reconnaissance survey of the identified river basins in Cebu, Bohol and Negros Oriental.