THERESA .1. MATRIC WATER-CONTENT MEASUREMENTS THROUGH-THICKNESS VARIATIONS IN VERTISOLS

It is difficult if not virtually impossible to measure the hydraulic c haracteristics of vertisols. Cracking of the soil makes the usual sens ors lose contact. Changes in the water content during the normal shrin kage phase can be calculated from vertical deformation measurements if the ratio between horizontal and vertical deformation components is k nown. THERESA is a new type of transducer for measuring the thickness of soil layers, It has a small diameter so that bias due to crack indu ction is delayed to avoid affecting water extraction by roots. Average s can be obtained from multiple sampling when spatial variability of t he soil is high. An equidimensional shrinkage model is used for calcul ating water contents. Water contents during normal shrinkage were accu rately estimated under the hypothesis that structural pores (which wer e assumed to remain rigid during the structural shrinkage phase) becom e constricted in proportion to shrinkage. Normal shrinkage, which is a ssociated with monotonic drying, seldom lasts very long in the field: it is interrupted by structural swelling at the slightest rainfall eve nt. THERESA only measures the amount of water which is associated with deformation and which is referred to as matric water.