| The inland, mountainous
marginal areas (land abandoned by farming and colonised by shrubs) of the
Iberian Peninsular, Spain, generally receive a higher rainfall than the coastal
areas (Lazaro and Rey, 1991) and may store water after cold season (autumn and
winter) rainfall. By measuring runoff, change of soil water content and
rainfall, this study tests the hypothesis that two shrubs on two sites on
abandoned land do not use all the water available after cold season rainfall.
One site was on an upper alluvial slope dominated by Anthyllis cytisoides and
the other on a lower alluvial slope dominated by Retama sphaerocarpa. The
root systems of A. cytisoides and R. sphaerocarpa penetrate to 3 m
and 20 m, respectively. A. cytisoides senesces during the dry season and
R. sphaerocarpa is evergreen. The water balance is dominated by high actual
evapotranspiration (ET), which is limited by rainfall. Reference
evapotranspiration was high; runoff was low and soil water storage occurred
above 2 m depth. ET and water storage were highest under A. cytisoides shrubs.
Runoff was lower on the ‘Anthyllis’ site. The spatial variability of soil
water is high and the problems of its measurement are discussed. The quantity of
rainfall infiltrated was greater under shrubs than grass-areas, suggesting that
shrub roots facilitated preferential flow. The growing season of
A. cytisoides began when water was available in the upper soil layers and
senescence occurred when the upper soil layers dried to less than 4% water
content. A. cytisoides, therefore, relies on water from these
layers. The main growth of R. sphaerocarpa occurred when the upper soil
layers were relatively dry, so that R. sphaerocarpa must extract water
from deeper layers. Results suggest that A. cytisoides accumulates
rainfall and runoff and directs water to lower layers for later use, while
R. sphaerocarpa extracts water from deeper soil layers. By mid-summer both
shrubs had extracted all the available water accumulated in the upper soil
layers from cold season rainfall.
Keywords: water balance, neutron probe, patchy vegetation,
mosaic vegetation, Spain, semi arid, Anthyllis cytisoides,
Retama sphaerocarpa |