Monday, January 9, 2012

disconnect between forecasts and vulnerability maps

After storm Sendong and the landslide in Compostela Valley, several maps propped out in the open. Experts flaunt their respective vulnerability and hazard maps.  LGUs supposedly knew this and were blamed for not having implemented precautionary measures to avert these disasters. I guess much have been said about poor governance and the biophysical vulnerabilities we are in. In this case, I will try to digest the relevance of these maps and try to analyze why despite all of these information, we are caught unaware when disaster strikes.

Flood maps supposedly showed inundation areas once 1-meter, or 2 meter flood levels come rushing in. Maps are good for planning. They give us idea of the spatial dynamics answering the important question of "where disaster may strike". But what these maps fail to say is "when disaster may potentially strike". Addressing temporal dynamics is also an important consideration in addressing disasters.

I think there is a disconnect between our appreciation of maps and digesting forecasts. What I mean is that we know where areas can be inundated by a 1-m flood stage but we do not know when this flood level may occur. At what rainfall can we say that 1-m flood stage may occur. For example, 180 mm would be destructive for CDO and Iligan. This would mean that rainfall forecast beyond 180 mm has to be watched out for CDO and Iligan (please see my previous article for Iligan: how rainfall can save from flooding). Every watershed respond differently, so preparedness would be different. For example, 180 mm would be minimal if situated in the Cagayan River in Luzon but would be destructive for Cagayan river in Mindanao. So communities have to be careful in listening to forecasts. Previously, I called for revising our alert system by focusing mainly on rain instead of wind (typhoon warnings: switching from wind to rain). This time, I believe it is high time to categorize our watershed and other water bodies that correspond to its response to rainfall volume, intensity, and duration.     

For example, a watershed can be categorized as high risk if floods may occur at very small rainfall amounts. Conversely, areas which can absorb high volume of rainwater can be categorized as low risk. It is not enough to say where areas can be flooded by a certain flood height but also when this height could possibly occur given rainfall amounts. I longed for weather forecast that states, "we expect rainfall up to 50 mm therefore areas under category A should be vigilant". I guess stating just "low lying areas" is not enough to warn because other low-lying areas may not have the topography vulnerable to floods.

In categorizing watersheds, topography and land use has to be considered. Areas with the impounding potential and "choked" topography would be different from really flat areas. Cultivated areas would respond differently from closed forest. Our current hazard maps are static in nature and does not into account changes in land cover. Hazard maps have to be revised as often as possible to reflect actual land use. Actually, maps generated ten years ago may not be applicable today because substantial land use change might have occurred during this duration. On the other hand, if vulnerability maps were based on historical hydrological data, it may not be able to capture future scenarios brought about climate change.       

There is a substantial stride in climate forecasting in recent times. For example, it has been predicted that above than normal rainfall is expected in Mindanao for December, January and February (La Nina). "Above than normal rainfall" may not sound alarming. But we need to understand that DJF period is when floodings "normally" occurred in Maguindanao, Caraga, and Davao. In this case, this would mean "above normal flooding".

My point here is that forecast is nothing if we do not reflect that with the vulnerability on the ground. We need to repackage our alert systems enough to warn. Vulnerability maps are nothing if we cannot connect that with forecasts. PAGASA, LGUs, and the so-called climate experts should try ways on how to connect their information that is understandable by the common tao. Please do not flood us with bytes, grids, pixels and weder-weder lang yan.

1 comment:

  1. My complements to you is long overdue. I've been following your words of wisdom in relation to disaster preparedness ever since your hydrological-flooding discussion in relation to Typhoon Sendong was posted. What you shared are precious jewels. Thank you very much. You have been a great help to us.