Dark Island heath (Ninety-mile Plain, South Australia). V. The water relationships in heath vegetation and pastures on the Makin sand

Abstract

Heath vegetation shows a major flush of growth during the mediterraneantype summer season, a time when calculations of the soil moisture storage by the techniques of Thornthwaite (1948) or Prescott, Collins, and Shirpurkar (1952) indicate that severe drought conditions should oocur. Monthly observations on the moisture status of the Makin sand under heath vegetation and, for comparison, under various pastures are therefore recorded.

The problems of obtaining an accurate water balance-sheet for such a heterogeneous vegetation as the heath are discussed. Difficulties in the use of the various techniques for measuring soil moisture in sand, which has a low pF of 1.85 at field capacity, are enumerated.

The following relationships were found between the evapotranspiration index (I_tr = E_tr / E_w^0.75) and the available water (W). These data were calculated for 6 ft of sand.

(i) Heath vegetation (10–14 years old) log (2.4–I_tr) = 0.420–0.0383 W
(ii) Heath vegetation (burnt) log (2.4–I_tr) = 0.461–0.0380 W
(iii) Oenothera odorata Jacq. pasture log (2.4–I_tr) = 0.395–0.0269 W
(iv) Medicago sativa L. pasture log (2.4–I_tr) = 0.390–0.0270 W
(v) Ehrharta calycina Sm. pasture log (2.4–I_tr) = 0.400–0.0339 W

From these equations the mean monthly quantities of rainfall which may be stored in 6 ft of sand under the various treatments described were calculated. Drought conditions are shown to occur in December and January, but are relieved in the later months of summer. Even if the stored moisture below 8 ft is considered, the soil moisture status would be expected to be just sufficient to maintain the vegetation in a "dormant" state, and yet the major growth of the heath vegetation occurs at this time.

The calculated mean annual values of I_tr range from 0.53 to 0.60 for these perennial communities. Close approximations to the actual soil moisture status can be obtained by substituting these values for K in Prescott's formula for potential evaporation, i.e. E_tr = K x E_w^0.75.

Supplementary data on transpiration, growth, and the root systems of the pastures are also included.