Drought Definition

As in other Mediterranean countries with large arid or semiarid areas, droughts in Spain are difficult to evaluate and quantify and thus are difficult to define. Many definitions are used, and often it is not clear when a drought situation has started or finished or even if it has existed. In some large river basins, the definition of a drought is based on simple rainfall statistics. For example, for the river Ebro, one of the largest rivers in Spain, a dry period starts according to Spanish law, "when rainfall amounts in two consecutive months within the series are lower than 60% of the average rainfalls for these months." For the river Guadiana, "a situation of drought occurs when the sum of rainfalls registered during the 12 preceding months is lower than those registered in 75% of the cases within the period analyzed." In some cases, the definition of drought is based on the relationship between supply and demand. For instance, in the Guadalquivir River basin, the following definition is used: "a situation in which the resources accumulated are insufficient to satisfy the demand." In the river basin Norte III* (one of the wettest in Spain), the experience of previous droughts must be taken into account to plan water supply facilities. The Norte III hydrological plan points out "the works have to be planned in accordance with the demand on the basis of the droughts that occurred in the 1941-43 and 1989-90 periods, without admitting any errors and considering that there are sufficient resources for such purposes within the scope of the plan." This plan also defines "dry year" as one in which the yearly cumulative flow is half of the average and "very dry year" as one in which the cumulative flow amounts to 75% of the flow registered in a dry year, or slightly more than 35% of the yearly average cumulative flow.

Regarding drought definition, an interesting case refers to the provisions about emergency situations included in the treaty between Spain and Portugal on transboundary river basins. This agreement, known as Albufeira, was signed in Albufeira, Portugal, at the end of 1998. It establishes that in case of drought situations, Spain will not be in breach of the treaty for failing to maintain the quantity or quality of the discharges fixed in the agreement as minimum inflow to Portugal for standard meteorological conditions. In a drought situation, it is understood that the first priority should be to maintain the water supply for urban uses and, in consequence, other requirements of the treaty would be temporarily repealed. Drought situations are defined on the basis of a referenced precipitation calculated for a specific time period in each river basin using data from only two or three rain gauges through different weights that are applied at each control point. This precipitation is compared with a percentile of the mean for the same period. Tables 1 and 2 explain the drought definition process proposed for the river Miño.

* Norte river basin is divided into three subbasins for administrative purposes: Norte I, Norte II, and Norte III.

Table 1 Step 1: Calculation of the Mean (M)a and Reference (R) Precipitation

Weights of Each Control Rain Gauge Point for the Precipitation

River Basin Stations Assessment

Orense 47%

Ponferrada 23%

a Calculated for the period 1945-46 to 1996-97 and updated every 5 years.

Table 2 Step 2: Identification of a Drought Situation

River Basin

Control Point (along border between Spain and Portugal)

Minimum Discharge to Be Ensured by Spain along the Border with Portugal (millions m3/year)

Miño Salto de Freira 3700

Starting Date for Derogation Period is July 1 if ...

R ( cumulative rainfall from October 1 to July 1) <70% M

(cumulative rainfall from October 1 to July 1)

Ending Date for Derogation Period is the Following Month to December if .

R (cumulative rainfall from October 1 to July 1) >M (cumulative rainfall from October 1 to July 1)

The lack of a universally accepted definition for drought has consequences that go beyond academic discussions. For example, as will be indicated in the following section, Spanish water regulations allow river authorities to adopt emergency measures, including occupation of private lands or stopping specific uses such as irrigation, if a drought situation has been declared.

Continue reading here: The Experience of the Drought

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