Water losses through paddy bunds: Methods, experimental data and simulation studies

Autor: Janssen, M.; Lennartz, B.
In:

Journal of Hydrology

Auflage: 369
ISBN: 0022-1694
Seite: 142 - 153
Jahr: 2009

Einordung:
Institut: Professur Bodenphysik
Teilschwerpunkt: Umweltgerechter Stoffhaushalt

Abstract:
Irrigated rice landscapes increasingly experience water scarcity, and the bunds surrounding paddy rice fields (also referred to as dikes, banks or levees) have been pointed out as possible causes for low water productivities. Yet, detailed experimental data to confirm this assumption is missing, as well as an identification of the most decisive factors for bund infiltration. The objective of this study was therefore to quantify water fluxes through bunds in a terraced paddy landscape both experimentally and by numerical analysis. Two bunds with a respective age of 20 (young bund: BY) and 100 years (old bund: BO) were investigated at the end of the rice-growing season. Soil core samples revealed a compacted hard pan at 30 cm depth in both bunds. Hydraulic conductivity was very high and variable especially in the upper part of the bunds. For direct measurements of infiltration into the bund, a double-frame infiltrometer was introduced analogous to a double ring infiltrometer. Measured infiltration rates in bund BO at ponding water depths (PWDs) of 3, 5 and 7 cm amounted to 480, 270 and 550 cm2 d−1, respectively, and no cross-flow was observed. At bund BY, the infiltrating water resulted in an increasing water level on the lower field, and the cross-flow was 807, 4205 and 5506 cm2 d−1 at corresponding PWDs. However, no reliable infiltration measurements could be recorded. 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. Different infiltration rates in the investigated bunds could be mainly attributed to a lower bulk density and enhanced macroporosity in the young bund BY. The existence of a hard pan in a bund does not necessarily reduce percolation losses, as it might become perforated by macropores originating from bio-activity. A sensitivity analysis realized 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, while soybean cultivation, bund width and elevation difference only play a minor role. For each conductivity profile, there is a critical bund width underneath which cross-flow occurred. A detailed description of the bunds’ water fluxes needs to be included in hydrologic modelling of irrigated rice landscapes.

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