Auswirkungen landnutzungsabhängiger Bodengefügedynamik auf die präferenzielle Verlagerung von Wasser und gelösten Stoffen in `Paddy`-Reisfeldern in Abhängigkeit der Raumskala
Zusammenfassung der Projektergebnisse
Rice is the major staple crop in southeast Asia, where increasing water scarcity and raising awareness about water quality issues involve the need to reduce unproductive water losses and the export of agrochemicals. Percolation is responsible for the major part of water losses in paddy landscapes and is strongly influenced by the soil structure, which is highly dynamic in the alternatively submerged and desiccated paddy soils. The general objective of this project was therefore to analyze the interrelation between water and solute fluxes, soil structure dynamics and soil and water management in paddy rice fields. The investigations were carried out in a small watershed in Jiangxi Province, China, on three paddy fields which had been used for rice cultivation for three, 20 and > 100 years. The infiltration rates (measured with double-ring infiltrometers) demonstrated a strong dependence on the age of the field. Puddling reduced the percolation rate about 35-fold after 20 years and 175-fold after 100 years of paddy cultivation, confirming the importance of maintaining an undisturbed, permanent plough pan in order to increase water use efficiency. Lateral flux experiments in the topsoil revealed that horizontal preferential flow drove water and solute fluxes in the cracked topsoil of paddy fields over a flow distance of 50 cm, indicating high water losses during land preparation even with an existing plough pan. Lateral preferential transport of water and solutes towards macropores in the plough pan or towards the bunds is facilitated. The bunds surrounding paddy rice fields have been pointed out as possible causes for low water productivities. Two terraced bunds with a respective age of 20 and 100 years were therefore investigated both experimentally and by numerical analysis. Infiltration rates measured with a double-frame infiltrometer revealed that the infiltration in the younger bund (characterized by a lower bulk density and a large macroporosity) was higher than in the old one. The measurements proved for the first time experimentally that the contribution of under-bund percolation is as large as that of within-field percolation. Tensiometers installed in the bunds during the infiltration experiments revealed a nearly instantaneous rise in pressure head after flooding throughout the profile in bund BY, indicating preferential water fluxes. In bund BO, contrarily, pressure heads underneath the hard pan rose slowly and with a time lag and thus show dominant matrix fluxes. A sensitivity analysis realised with HYDRUS 1.05 revealed that under-bund percolation is most strongly influenced by the hydraulic conductivity in the upper and middle bund. Ponding water depth and groundwater table are less important. Dye tracer experiments conducted in the same bunds demonstrated that the bunds were distinctly more vulnerable to preferential transport of solutes toward the groundwater than the actual paddy fields. The younger bund showed greater water losses than the old one, caused by a greater macroporosity, lower bulk density, and a less effective hard pan. Three successively operating transport processes could be distinguished: (i) dye solution infiltrated vertically into the field and laterally into the bund; (ii) the plough pan underneath the field and the hard pan in the bund then acted as flow barriers, and horizontal spreading occurred on top ofthe hard pan toward the outer bund and cross-flow to the adjacent field was observed; and (iii) macropore flow occurred through the hard pan and partly in the subsoil down to the groundwater level, initiated on top of the saturated hard pan and routed through earthworm burrows and root channels as well as shrinkage cracks in the younger bund. The compaction of a bund may thus significantly reduce infiltration rates but does not prevent preferenfial solute losses. In order to assess percolation losses at the scale of channel command areas, a simple deterministic model with stochastic elements was developed that takes into account the spatialtemporal variability of the plough pan's hydraulic conductivity Ks as a consequence of the highly dynamic soil structure. Results demonstrate that percolation losses increase in the following order: continuous soil saturation (CSS) < continuous flooding (CF) < mid-season drainage and intermittent irrigation (MD+AWD) < mid-season drainage and continuous flooding (MD+CF). The bunds contribute up to 54 and 17 % to total fluxes under CF and MD+AWD, respectively. Preferential water fluxes are responsible for the major part of water losses as soon as thorough desiccation causes the formation of shrinkage cracks. In the light of new governmental directives in which intermittent irrigation is propagated, the results of this research projects suggest that the temporal dynamic of soil hydrological properties of paddies is of critical importance and needs further investigations.
Projektbezogene Publikationen (Auswahl)
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2005. Bodenhydrologie Soil hydrology of paddy soils in SE China. Geophysical Research Abstracts 7, 06196
Janssen, M., Muth, A., Lennartz, B.
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2005. Bodenhydrologie von Paddy-Reisfeldern in Südost-China. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft 106, 33-34
Janssen, M., Muth, A., Lennartz, B.
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2005. Bodenhydrologie von Paddy-Reisfeldern in Südost-China. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft 107, 57-58
Janssen, M., Lennartz, B., Muth, A.
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2006. Water losses through bunds in paddy rice fields. Geophysical Research Abstracts 8,00863
Janssen, M., Lennartz, B.
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2006. Horizontal and vertical water fluxes in paddy rice fields of subtropical China. In: Horn, R., Fleige, H., Peth, S., and X. Peng. Soil management for sustainability. Advances in Geoecology 38, 344-354. Reiskirchen
Janssen, M., Lennartz, B.
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2006. Vertical water losses in irrigated rice landscapes - Model development. Geophysical Research Abstracts 8, 01921
Lennartz, B., Janssen, M., Wöhling, T.
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2007. Horizontal and vertical water and solute fluxes in paddy rice fields. Soil Till. Res. 94, 133-141
Janssen, M., Lennartz, B.
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2007. Wasserflüsse in Dämmen von Paddy-Reisfeldern. In: Miegel, K., Trübger, E.- R., and H.-B. Kleeberg. Einfluss von Bewirtschaftung und Klima auf Wasser- und Stoffhaushalt. Forum für Hydrologie und Wasserbewirtschaftung 20
Janssen, M., Lennartz, B.
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2007. Wasserflüsse in Dämmen von Paddy-Reisfeldern: Eine Farbtracerstudie. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft 110, 107-108
Janssen, M., Lennartz, B.
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2008. Characterization of preferential flow pathways through paddy bunds with dye tracer tests. Soil Sci. Soc. Am. J. 72, 1756-1766
Janssen, M., Lennartz, B.
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2008. Ecological safe management of paddy landscapes. Soil Till. Res.
Lennartz, B., Horn, R., Duttmann, R., Gerke, H.H., Tippkötter, R., Eickhorst, T., Janssen, I., Janssen, M., Ruth, B., Sander, T., Shi, X., Sumfleth, K., Taubner, H., Zhang, B.
