CRACKING PATTERNS, BYPASS FLOW AND NITRATE LEACHING IN GREEK IRRIGATED SOILS

Bypass flow of water during irrigation by flooding, and associated leaching of nitrates, were investigated in sixty-three, 40 cm high undisturbed cores of nineteen calcareous alluvial Greek soils. Cracks conducting bypass water formed a network of polygons on the soil surface. The sizes of the polygons increased curvilinearly with clay content. Large polygons were associated with deeper cracking of the soil mass than the small polygons did. The size of the polygons decreased with depth in the profile. Upon irrigation cracks developed in dry soil were closed by swelling and by soil material falling from their walls. The closing was either complete or incomplete, the latter leaving a shallow U-shaped trace. Upon drying, cracks reformed along the U-shaped traces. Elsewhere, cracks developed at new places. The volume of bypass water showed a sharp initial decrease with time because of filling of the cracks and swelling, but remained significant even after 2 hours (the usual irrigation time) suggesting that a significant amount of the applied water may move to the subsoil without wetting the surface soil. Bypass flow showed a marked decrease with increasing initial soil moisture. The amounts of nitrate leached by bypass water were rather small, ranging from 0.01 to 1.8% of nitrates present in the soil cores.