RAINFALL PARTITIONING BY MEDITERRANEAN VEGETATION AND ITS EFFECTS ON SOIL
WATER BALANCE
Pilar Llorens1, Aleksandra Muzylo1, Jérôme Latron1, Susanne Schnabel2, Francisco Domingo3,
Yolanda Cantón4, Francesc Gallart1
1Institute
of Environmental Assessment and Water Research (IDÆA), CSIC. Barcelona, Spain.
pllorens@ija.csic.es, amuzylo@ija.csic.es, jlatron@ija.csic.es, fgallart@ija.csic.es
2Department of Geography. University of Extremadura. Cáceres, Spain. schnabel@unex.es
3Arid Zones Experimental Station (EEZA), CSIC. Almería, Spain. poveda@eeza.csic.es
4Department of Soil Science and Agriculture Chemistry, University of Almeria, Almeria, Spain.
ycanton@ual.es
Rainfall partitioning by vegetation plays an important role affecting the water balance at local and
catchment scale, because vegetation canopies modify both the redistribution of incident rainfall and
the evaporation of intercepted water.
When rainfall drops strike the canopy elements, part of this water evaporates directly (rainfall
interception) and the rest passes through the canopy and reaches the soil by stemflow or throughfall.
These two processes have a large degree of temporal and spatial variability and consequently, may
lead to significant changes in the volume and composition (solutes) of water that reach the ground.
For this reason rainfall partitioning, and its temporal and spatial variability, have a fundamental
importance on the water and solute inputs to the understory vegetation and the soil. It is generally
considered that throughfall affects the surface soils horizons and stemflow, channelled by branches
and stems can reach deeper soil layers and remain available for the roots.
In Mediterranean conditions mean relative throughfall in trees is higher (79%) than in shrubs (49%)
and is less variable. Throughfall of Mediterranean tree species is similar to that observed in temperate
conditions. For shrubs, studies are scarce and in addition this flux is highly variable in these species.
For both vegetation types, stemflow is under-represented in the literature, not only in the
Mediterranean area but also in the other climatic regions.
This work investigates the effect of rainfall partitioning on soil water content in three Mediterranean
study areas covering a wide climatic gradient and different vegetation species. From Northern to
Southern Spain the studied areas are: The Vallcebre research catchments (Pre-Pyrenees, 42º 12’N,
1º 49’E) where the species studied are Pinus sylvestris and Quercus pubescens, Parapuños research
catchment (Cáceres, 39º 35’N, 6º 5’W ), a wooded rangeland with Quercus rotundifolia and annual
grasses in open areas), and the Tabernas experimental Area (Almería, 37º 0’N, 2º 26’W) with disperse
shrubs (different species) and a mixture of annual plants and biological soil crusts in open areas.
Mean annual rainfall ranges between 862 mm in Vallcebre and 235 mm in Tabernas.
For tree covers the dominant flux was throughfall with a similar rate for the 3 studied species (7377%), being stemflow only a small part of the bulk rainfall for all tree species (1-3%). As a
consequence, interception loss for the studied trees represented about 24% of bulk rainfall. On the
other hand, for the studied shrubs, measured throughfall (35-70%) as well as stemflow were highly
variable between species.
Superficial water content (0 to 15-20cm) was measured in Vallcebre under forest and in neighbouring
clearings (annual grasses). In Parapuños and Tabernas, as vegetation is scattered, soil moisture was
measured in open areas and under individual trees or shrubs respectively.
In Vallcebre, soil water content was on average drier under forest (0.21 cm 3cm-3, CV=6%) than in the
neighbouring clearing (0.29 cm 3cm-3, CV=7%). In the same way, in Parapuños soil water content was
drier under oak trees (0.19 cm 3cm-3, CV=11%) than in open areas (0.24 cm 3cm-3, CV=9%). Contrarily,
in Tabernas soil water content was wetter under shrubs (0.16 cm 3cm-3, CV=52%) than in open areas
(0.13 cm3cm-3, CV=42%), although with higher variation coefficient.
Driest soils below forest covers as in Vallcebre, or even in sparse covered areas as the Parapuños
catchment, could be explained by the dominant role of rainfall interception and transpiration. In
Tabernas, soil moisture usually remained higher under shrubs than below open areas except for some
small rainfall events. This is mainly related to the fact that, shrubs architecture funnels precipitation
towards the soil through stemflow. Moreover, individual shrubs may also act as sinks for overland flow
generated in open areas, promoting infiltration.
Keywords: rainfall interception, throughfall, stemflow, soil water content, Mediterranean areas.