Cagayan basin runoff study

Where the rain goes

Change the storm and the ground in the Cagayan de Oro river basin, and see how much of the rain runs off.

An interactive study that runs in your browser. It uses a simple, explained model with illustrative starting numbers. It is not a forecast and not a live system.

Make a storm
Forested mountain range rising above farmland and scattered houses on the Bukidnon plateau, under a clear blue sky.
Bukidnon uplands: forest on the range, farmland below it.

Make a storm

Set how much rain falls in one day, how wet the soil already is, and what covers the ground. The two tubes share one scale: the rain that fell, and the runoff that left.

Cross-section of the basin from ridge to bay A hillside runs from a forested ridge on the left down to a river mouth and a bay on the right. Rain falls on it. Water runs along the surface, soaks into the ground, and enters the river. Two glass tubes on the right show rain and runoff on the same scale.

Runs off 0 mm
Stays on the land 0 mm Soaks in or is held by plants and puddles
Across the basin 0 million m³ If the rain fell evenly on about 1,370 km²

Schematic drawing, not to scale. The tubes and the bar show the model's numbers; the river and the plume only echo them.

Rain in one day
120 mm

Washi (local name Sendong) was a tropical storm. 181 mm is the reading its heaviest day gave at the PAGASA gauge in Cagayan de Oro; gauges in the uplands reported more.

Soil before the rain

Soaked ground has little room left, so more of the next rain runs off.

What covers the ground

Moving one share rescales the others so the basin always adds up to 100%. The starting mix is illustrative, not a land-cover survey.

Same rain, different ground

Each bar is the whole day’s rain. The brown part ran off. The top four show the basin as a single cover; the last bar is your own mix.

How the number is made

This is the curve-number method, a standard hydrology tool for turning one day of rain into one day of runoff. It has five steps, and the slip beside this text works them out for the storm you have set.

  1. Each land cover gets a curve number, CN. The higher it is, the less rain the ground can take. These are the standard table values for ground in good condition on medium-draining soil.
  2. Soil wetness shifts the curve number: soaked ground acts like a higher number, dry ground like a lower one.
  3. The ground's holding capacity is S = 25,400 ÷ CN − 254, in millimetres. The first fifth of it is lost before any runoff starts.
  4. Runoff for the day is Q = (P − 0.2 S)² ÷ (P + 0.8 S), where P is the rain. If P is no more than 0.2 S, nothing runs off.
  5. Average the four runoffs, weighted by how much of the basin each cover takes up.

Your storm

    Basin runoff, weighted0 mm

    From the ridge to the bay

    The plate above draws the slope in four bands. These are the kinds of ground it stands for.

    Forested mountain range above farmland, Bukidnon.
    Ridge and forest. Trees and leaf litter hold the first of the rain. In the model, forest has a curve number of 55, the lowest of the four.
    Close view of pineapple plants with fruit growing among long spiky leaves.
    Row crops. Pineapple plants near Camp Phillips, Manolo Fortich, Bukidnon, on a neighbouring plateau. Bare soil between the plants lets more rain run off, so the model gives row crops a curve number of 78.
    The Cagayan de Oro river flowing under a bridge, with buildings on both banks.
    River and town. The Cagayan de Oro River at Carmen, 2023. Roofs and roads send most of a heavy rain straight to the drains, so the model gives town ground a curve number of 92.
    Satellite view of Cagayan de Oro and Macajalar Bay. Brown river water spreads into the dark blue sea from the river mouths.
    Bay. Satellite image after Tropical Storm Washi, December 2011. Silt-brown river water spreads out into Macajalar Bay. The plate's plume grows the same way as runoff rises.

    What this leaves out

    It answers one question well enough to see the shape of the answer. It cannot tell you whether a river will flood.

    • One day, one total. There is no clock. It gives the depth of runoff, not when the flood peak arrives or how high the river rises.
    • Even rain. The same rain falls on every corner of the basin. Real storms are patchy: on the night of Washi, upland gauges reported more than the city gauge.
    • Borrowed numbers. The curve numbers come from standard tables written for small catchments, and the soil is assumed to be one medium-draining type. Nobody has calibrated them to this basin here.
    • Invented mix. The starting land-cover shares are illustrative. They are not measured from this basin.
    • No channels, slopes or river banks. Bamboo planted along the river, levees and floodways are not in the model.

    For warnings and forecasts, use PAGASA bulletins and your local disaster risk reduction office. This page is a study aid.