Field determination of water withdrawal from soil by plant roots as a function of depth and time.The water flow through roots as a function of depth and time can be determined in homogeneously rooted, level soils with closed plant canopies, provided the total vertical water flow and the capillary water flow are known for the soil profile. Theory, measuring methods and some results on water withdrawal of wheat (fig. 2–4) and sugar‐beet (fig. 5) roots on a loess grey‐brown podsolic soil profile underlain by gravelly sand (fig. 1) are described.The time averaged rates (2–7 day periods) for water extraction by roots are up till about 12 × 10−3documentclass{article}pagestyle{empty}begin{document}$ frac{{{rm cm}^3 {rm water}}}{{{rm cm}^3 {rm soil} cdot {rm day}}} $end{document}During the growth period variations of the depth of the effective root zone and of the extraction rate in distinct layers were found (fig. 6 + 7). A capillary rise from the subsoil into the effective root zone exists for some time (fig. 3); seepage water within the lower root zone causes a strong increase of extraction rates in these depths (fig. 4). The relative water extraction rate at the depths of 15 and 25 cm decreases with increasing soil water suction in the range between 150 and 104c
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