THERESA .2. THICKNESS VARIATIONS OF VERTISOLS FOR INDICATING WATER STATUS IN SOIL AND PLANTS

THERESA (Transferts Hydriques Evalues par le REtrait des Sols Argileux ) is a method for estimating the water content of vertisols in the fie ld by measuring the vertical deformation of the solid. Deformation is controlled by changes in the water content of the day matrix, and only the matric water component of the total water content can be determin ed with this method. The relationship between matric water storage or reserve (Sm and Rm(i)) calculated from deformation data and three indi cators of water status in the soil and plants were studied, so as to f ind out more about the actual meaning of this parameter in terms of wa ter uptake by plants. The three indicators used were: total water stor age and reserves (S and R(i)) which control the amount of water availa ble, relative transpiration (Ep/ETm) which is the outcome of the balan ce between water availability and plant requirements, and the growth r ate of an organ (Delta L/Delta t) which indicates how water stress aff ects plant growth. Evaluating matric water contents from changes in th e thickness of soil layers (measured with THERESA transducers) was tes ted in three settings: in grassland from 1985 to 1988 and during two s ugar cane growth cycles from 1989 to 1991. By measuring vertical solid displacements and the gravimetric water content, the soil shrinkage c urve in the field and changes in the thickness of soil layers were ava ilable. These were then used for removing the bias in water storage ca lculation introduced when the changes in bulk density and thickness ar e ignored. Matric water contents measured with THERESA corresponded to a low availability domain. Relative transpiration (Ep/ETm) and stem g rowth rate (Delta L/Delta t) decreased significantly as soon as the ma tric reserve was drawn from. These results give confirmation that matr ic water contents measured by THERESA are an appropriate indicator for rationing water and for monitoring irrigation in swelling clay soils.