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Na Borges catchment
Location
The Na Borges catchment extends over the northeastern part of the island of Mallorca (Spain; 39º 38’ 14”N 3º 09’ 11”E) where it is the second largest catchment, encompassing an area of 319 km2. It collects water from the southeastern parts of the Central Ranges and from the western slope of the Llevant Ranges. The altitude ranges from 300 to 500 m a.s.l. in the catchment headwaters, where the average slope of the channels is 8%. Downstream, the main channel has a very gentle average slope (0.3%) along its 26.8 km course to the sea.
Geology
Na Borges is a groundwater dominated river where the surface water/groundwater interaction is controlled by its hydrogeological and geomorphological characteristics and their modification by human impact. As a result, different streamflow regimes can be observed in the catchment. The headwaters of all sub-catchments and the tributaries that drain the Llevant Ranges and Marina de Petra are ephemeral due to the high degree of fracturing, fissuring and karstification, which favours infiltration and percolation through perched karstic aquifers unconnected to the channels. The lower flat areas (i.e. Pla de Son Pou) are covered by deep soils developed on the Quaternary alluvial sediments which are underlain by impermeable Lower-Middle Miocene sediments. These alluvial sediments form a shallow unconfined aquifer where groundwater flow sustains an influent discharge, but the high evaporation rates and low precipitation during the summer result in an intermittent regime. The first 5 km of Na Borges Canyon consists of impervious Lower-Middle Miocene strata that receive a perennial regime of influent flow from karstic springs draining the reefal Upper Miocene tabular platform. Downstream, these strata are progressively replaced by porous Middle-Upper Miocene sediments. Consequently, transmission losses assume increasing importance along the remaining 12 km of the Na Borges Canyon, where the intermittent flows reflect both lithological controls and the hydrometeorological conditions in this Mediterranean area.
Vegetation/Land use
The catchment is predominantly rural, with a population of 47,458 inhabitants in 2001. More than 80% of the land is used for rainfed agriculture. Agriculture has modified the lower areas of the catchment, with subsurface tile drainage covering 12% of the catchment, mostly in the Pla de Son Pou. This drainage increases the flow to the Na Borges River from the shallow unconfined aquifer above the impervious Lower-Middle Miocene strata.