No abstract

SUMMARYTwo l a r g e l y s i m e t e r s were i n s t a l l e d i n t h e s o i l o f t h e Hanford R e s e r v a t i o n t o g a t h e r d a t a on t h e a c c u m u l a t i o n and p e n e t r a t i o n o f m e t e o r i c w a t e r . T h i s r e p o r t d e s c r i b e s t h e c o n s t . r u c t i o n d e t a i l s o f t h i

Deep water flow was evaluated in a Washington desert environment using hydraulic conductivity and potential gradients. Thermocouple psychrometers and temperature transducers were installed to depths of 94 m in the soil profile and used to measure the potential gradients. The hydraulic conductivity was calculated using a modified Millington and Quirk equation and the soil moisture characteristic curve. The thermal fluid diffusivity was calculated and used to estimate flow induced by thermal gradients. Under the conditions studied, a more refined analysis of the thermally induced flow is required to give a definite answer as to the direction of flow. It was concluded, however, if flow existed at this location, it was less than 1 cm/year.

A technique for controlling water content in the root hair zone of growing plants has been developed which involves growing plants on a fine screen placed over soil in such a way that only root hairs can extend through the screen into soil beneath. Gamma‐ray attenuation is used to monitor the supply of water in the root hair zone. By means of controlling either the length of a hanging water column or the rate of water supply, the rate of water flow to the soil lamina beneath the screen can be so adjusted as to just offset the loss due to transpiration. A predetermined soil water content can be maintained to within 0.015 cm3/cm3 in the root hair zone of a growing plant.Plant growth was affected by soil water potential. Reduction in elongation rate of corn (Zea mays L.) leaves was observed at matric potentials higher than −1 bars. Elongation essentially ceased at matric potentials of about −22 to −30 bars within the root hair zone. A large water content gradient existed adjacent to the root hair zone and extended to a depth greater than 10 mm.

The energy status (potential) of water can be used to calculate water flux. Thus, field measurement of water potential is superior to conventional gravimetric soil moisture content measurements for dynamic soil water studies and is useful for steady state analysis of soil water transport. Data presented show that soil thermocouple psychrometers give sufficiently sensitive, in situ measurements of water potential to apply energy concepts to dynamic soil water movement. Thus the soil thermocouple psychrometer can be used to obtain soil water potential information needed to determine the direction of water flow and the magnitude of the driving force. This technique is not limited to studies of the penetration of rainfall in the arid region but also has application in evapotranspiration studies, and might be adaptable to such studies as groundwater hazards of sanitary land fills.