MethodologyHydrometryHydrometric StationHydromatric station is a station located on a riverside that measures discharge, and other flow parameters. In a water basin system, it is used as a control point.In order to achieve good results, the hydrometric station location should follow some conditions. According to Boyer (1964), and Horst (1978), there are some conditions which should be fulfilled, as follow:a. The station should be easily accessible in any conditions,b. The location should be at the straight stream flow, with the minimum straight stream length is 3x stream width,c. The location should have a stable cross section, which means the relation between water level and flow discharge does not change,d. There is no backwater occurs at the location,e. There is no obstacles such as aquatic plants, and rocks.Water Level ObservationWater level is a parameter which is very important to be observed. Water level is different with water discharge, because it can be observed directly with simple direct observation, which make it very important to be observed. There are 2 different observations: automatic and manual. Manual observationStaff gauge is used for water level observation manually. Staff gauge is a graduated rod, made of wood or aluminium. The rod should be placed over a strong foundation so the position does not change. It is also important to have the rod easy to read.There are several different placement of rods, depends on the situationAutomatic observationAutomatic Water Level Recorder (AWLR) is an instrument used for automatic observation. It records water level automatically in every specific time, for example every minute.Stream Flow DischargeStream flow discharge cannot be observed directly. In order to achieve the discharge the area and the velocity are needed. Both parameter, however can be measured directly at the river. The area and the velocity then can be used for calculating the discharge by using this formula:Q = V x AWith:Q:dischargeV:velocityA:areaAreaThe area can be obtained after the cross section measurement is done. River cross section is measured perpendicular to the flow direction. The cross section measurement is done by measuring the depth at some points where the distance between them usually the same. After the measurement is done, the cross section can be divided into some subsections. The subsections are the area between points with the same width on all subsections.Velocity

The stream flow velocity can be observed using current meter. Current meter is a propeller device. The velocity is obtained as a function of the current meters rotation within a specific time. Current meter only measures velocity at a point, however the velocity is different at all points in the river. in order to obtain the average velocity the measurement should be done several time at different subsections. At every subsections the measurement can be done using three different methods:1. Only one measurement done. The measurement is done at 0.6 depth. The average velocity is the velocity obtained from the measurement.2. Two measurements done. The measurement is done at 0.2 depth and 0.8 depth. The average velocity is (V0.2+V0.8)/2.3. Three measurements done. The measurement is done at 0.2 depth, 0.6 depth, and 0.8 depth. The average velocity is {(V0.2+V0.8)/2+V0.6}/2.Besides the total measurement done in every subsections, the measurement can also be done using 4 difference ways depending on the river situation:1. Measurement on the river without special tools. It can be done on Shallow river.

2. Measurement using a boat. It is done when the river is deep, wide, but the flow is not very fast.

3. Measurement on the bridge. It can be done if only there is a bridge. Not like the measurement on a boat, the measurement on a bridge can also be done on the river with fast flow.

4. Measurement using a cable train. This measurement should be done when the river is deep, wide, and the flow is fast, but there is no bridge.

Rating CurveAfter the discharge can be calculated, the rating curve should be calculated. Rating curve is a curve that describes the relation between water level and discharge. Rating curve is needed to predict the discharge using the water level, which is the easiest to observe every time. The rating curve can be obtained using manning equation.

With:V:velocityR:hydraulic radiusA:slopeThe hydraulic radius is depended on the water level, so the rating curve can be obtained after substitute the velocity in the discharge formula using the manning equation.