Everything runs smoothly in years of normal rainfall. But when the region gets below-normal rainfall-as is typically the case during an El Niño–the situation gets contentious.Angat’s water allocations, as well as those of reservoirs throughout much of the world, are guided by upper and lower ‘rule curves’, or thresholds. When the water level in the reservoir is above the upper rule curve, there is generally less tension. The reservoir meets both the domestic water supply and irrigation demands and hydropower can be generated. If the water level falls somewhere between the two thresholds, domestic water and irrigation needs can be met, but power generation is limited. But when the level drops below the lower threshold, trouble starts to brew. In the case of Angat, domestic water needs are always met first, and irrigation water is released only if the water level is above 180 meters, the reservoir’s minimum operating level.

In 1997 and 1998, for example, the reservoir had to stop releasing irrigation water because of low levels. Crops withered, farmers fumed. The magnitude and duration of the cutoff could have been minimized, however.

The problem with most rule curves is that they are static and very conservative. They’re usually based on long-term historical averages, without taking into consideration climate variability and seasonal forecasts.

The IRI has worked with a number of government agencies in the Philippines over the last five years to develop a reservoir model for Angat that integrates seasonal climate forecasts into the current water management strategy. Why is this important? Our simulations have shown that integrating forecasts into the decision-making process not only reduces the risk of over-allocating during dry years, but also maximizes agricultural and hydropower production during the wet years, and hopefully avoid unnecessary hardships, such as the reneging of promised irrigation water after farmers have already committed to planting.

Of course, having a great model is only part of the solution. The water allocation process for Angat is extremely complex and notoriously political. I’ll be learning and writing more about this process as I meet with representatives from all sides of the issue in the next few days.

[Note: source material for this entry comes from IRI reports and other documents, most of which can be found on the institution's Asia and Water pages.]

Photo: Water meter, Intramuros, Manila. Francesco Fiondella

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