H43G-1699
A computational algorithm for hillslope partitioning from DEMs and digitized river networks
A computational algorithm has been developed for delineating and parameterizing hillslopes from digital terrain models of watersheds or relatively homogeneous hydrological units (RHHUs). The derived hillslopes can be characterized according to slope, plan shape, profile curvature, width function, and other parameters needed in hillslope-focused hydrological field and modeling studies. The procedure begins by dividing the RHHUs into either two or three hillslopes, the latter case representative of a headwater unit. The plan shape and profile curvature can be computed using either a binary or a fuzzy logic approach. The width function calculation integrates two different methods of interpolation, depending on the presence or not of a headwater hillslope. Different criteria, such as total area matching, are used to guide and assess the delineation process. In contrast to other partitioning algorithms (e.g., LandMapR software), the proposed method can be integrated directly into PHYSITEL, a watershed GIS, and can thus work with RHHUs derived from this tool, i.e., within a hydrological unit framework.
H43G-1700
Estimation of suspended sediment sources using Cs-137 and Pb210ex in unmanaged Japanese cypress plantation watersheds of southern Japan
To assess the suspended sediment sources in unmanaged Japanese cypress plantation watersheds, field measurements and fingerprinting of suspended sediment were conducted in the Shimanto River basin, southern Japan. For the fingerprinting, Cs-137 and Pb210ex were detected by gamma-ray spectrometry for the potential source materials in the surface soil of the forest floor, the stream bank and the truck trail, the mobilized sediment by raindrop impact and overland flow, and the suspended sediment in the stream channel. The Cs-137 and Pb210ex activities associated with forest floor materials were considerably higher than those of the stream bank and truck trail. The Cs-137 and Pb210ex activities associated with suspended sediment were found to vary with the sampling period. These results suggest that the suspended sediment can comprise materials from forest floor and/or landslide scar and that from truck trail and/or stream bank. The generally lower contribution from the forest floor in Hinoki 156 as compared to that from Hinoki 155 indicates that truck trail networks may be secondary sources of suspended sediment in unmanaged Japanese cypress plantations. Based on field measurements, overland flow occurred during rainfall events on the hillslope, eroded the surface soil on the forest floor, and transported sediment downslope. Overland flow on the truck trail networks may also transport forest floor sediment into the stream channel. These results indicate that the forest floor should be recognized as an important source of fluvial sediment in this watershed.
H43G-1701
Effect of Spatial Variation of Infiltration on Overland Flow Generation and Hydrological Processes in a Small Catchment covered by Japanese cypress plantation
We examined the effects of overland flow on storm runoff in a headwater catchment by investigating the spatial distribution of soil moisture and infiltration rate along a hillslope transect, Hortonian overland flow, stream discharge, and soil water potential. The study was conducted in a small catchment covered by 40-year-old Japanese cypress forest with a stand density of 3500 stem/ha. Overland flow observations were conducted on a hillslope plot (8 x 20 m) that extended from a ridge to the middle of a hillslope adjacent to the transect. Stream discharge was monitored at the outlet of the 0.43-ha catchment. We measured the infiltration rate 15 times over 20 months at each of three plots (0.3 x 0.3 m) installed in the upper, middle, and lower parts of the hillslope transect. The infiltration rate in the lower plot (average 1.37 mm/min) was significantly greater than that in the middle (average 0.90 mm/min) and upper (average 0.81 mm/min) plots. The relationship between the infiltration rate and soil moisture in each plot was negative, indicating the effect of soil water repellency on infiltration and overland flow generation. Likewise, the overland flow for storm events was greater during a dry period (May–July 2005: 332 mm of precipitation) than during a wet period (May–July 2006: 528 mm of precipitation). The storm runoff from the catchment showed the opposite tendency, i.e., it was greater during the wet than the dry period. During storm events in both dry and wet periods, stream discharge began to increase sharply when lateral subsurface flow formed groundwater at the bottom of the hillslope transect, whereas overland flow was responsive to rainfall intensity throughout the events. Thus, subsurface flow, rather than Hortonian overland flow, dominated storm runoff from the catchment, probably because overland flow on hillslopes infiltrated into the soil matrix lower on the slopes, where the infiltration rate was high. However, during short, intense storms under dry conditions, stream runoff increased, as did overland flow, without increases in soil water potential below 10 cm in depth, suggesting that Hortonian overland flow was the main contributor to storm runoff during these events.
H43G-1702
Influence of Vegetation and Management Conditions of Forest Cathcments on Their Runoff Characteristics
There is rising concern that devastation of planted conifer forests due to poor management might change runoff characteristics and provoke severer flood disasters in Japan. Planted conifer forests in Japan were planted from 1960s to 1970s as a countermeasure to building materials shortage. Domestic forestry products, however, lost out in the price competition with imported products after 1970s, so that forestry has stagnated in Japan, which causes the shortage of forestry workers to maintain planted forests. In this study, runoff variations for the same rainfall pattern observed at neighboring catchments with different conditions were compared so that influence of vegetation and management conditions of forest cathcments on their runoff characteristics was examined. Besides, a Long-and-Short Term (LST) model was calibrated for each of the objective catchments to know the difference in hydrological components as surface flow, subsurface flow, groundwater flow, infiltration and percolation. In three observation sites, eleven catchments were selected as objectives. The areas of the catchments vary from 0.17 to 36 ha. First, runoff characteristics between conifer and broadleaf forests was compared. The result clearly showed that total runoff depth and peak runoff for storm events were both larger in conifer forests than in the broadleaf catchments, and that total infiltration was smaller in conifer than in broadleaf catchments. The result supports the fact that natural forests are generally considered to have functions of water retention and flood control. Second, runoff characteristics between well and poorly managed planted conifer forests was compared. The result, however, did not show the clear difference. It was difficult to find correlation between forest management condition of catchments and total amount of each hydrological process during some storm events, so that the existence of forest management impact on runoff characteristics could not be identified only by this investigation. It is probably due to mutual interaction of the forest management effect with the other factors as topography, catchment area, soil characteristics, strongly affects runoff characteristics. These results suggest that more investigation is required.
H43G-1703
Overland Flow Induced by Snowmelt and Partially Frozen Conditions
The transport infrastructure is a vital part of the society, with high capital investments. Design of this system is therefore very important. Climate changes will increase the frequency of extreme precipitation events, floods and snow melt periods experienced by the infrastructure. According to initial analysis by the Norwegian transport sector these changes will affect road maintenance, emergency planning, design of new roads and infrastructure. Increased frequency of floods is expected to cause more closed roads because of insufficient and badly maintained drainage systems. Increased ground frost and ice formation on ground surface cause large increases in surface runoff during snowmelt. Recently, in Norway the ClimRunoff project has started with the focus on quantifying discharge of catchment areas draining towards roads. The first priority of the project is to create a model that can evaluate the run-off situations under spring situation (i.e. overland flow due to snowmelt and partially frozen soils). The model is tested on a well-defined catchment under autumn situation. Preliminary results of the model calculations will be presented, together with the challenges to alter the model to be able to calculate snowmelt and frozen soil conditions. In close cooperation with the Norwegian road authorities, areas with historical flooding events are selected, and current and future climate data will be used to analyse the infrastructure of the road construction. Together with a risk analysis of the vulnerability of the transport infrastructure the model will be used to create guidelines for road construction with respect to run-off and drainage that can account for changes in climate.
H43G-1704
Residuejams and their effect on Infiltration, Runoff and Dissolved Organic Carbon (DOC) in Furrow Irrigation Systems
Crop residue, which consists of small debris of different sizes, is an important resource in agricultural ecosystems. It plays a vital role in conservation tillage as a best management practice (BMP) for reducing runoff, sediment, nutrient and pesticide transport from irrigated fields. In furrow irrigation, the predominant irrigation method in the world, as irrigation or winter runoff water moves along a furrow, it lifts the unanchored residue and transports across the field. The complex interaction of multiple residue pieces (debris) with itself, the soil matrix, and the fluid cause jams to form along the furrow. The residuejams can be thought of logjams in fluvial rivers or channels which help in increasing channel roughness to reduce flow velocities and shear stress along eroding banks. The logjams also create a hydraulic shadow, a low-velocity zone for some distance upstream that allows sediment to settle out and stabilize. Similarly, the residuejams help in formation of catchments, which promote increased infiltration and settlement of sediments along the furrow. The infiltration and residue interaction with the soil-water influence the runoff, sediment, nutrient and dissolved organic carbon (DOC) export. The reduction of DOC export is critical to enhancing drinking water resources by reducing reactive sources of DOC that form carcinogenic by-products in the disinfection process. Hence, investigation of geomorphology of the residuejams is essential to understand their impact on infiltration, runoff and DOC concentration. This study focuses on the formation of residuejams and their effect on the infiltration, runoff and DOC concentration from 122 m long furrow plots with cover crop (CC), no-till (NT) and standard tillage (ST). These treatments (CC, NT and ST) were replicated three times using randomized complete block design and the plots initially, had 10, 32 and 42% of residue cover (sunflower residue on ST and NT; sunflower and wheat residue on CC plot), respectively. During the irrigation, the waterfront advance (for estimating infiltration), runoff and DOC concentration (at the tail end) were measured. After the irrigation, the location of the residuejams along the furrow and their dry weight were measured. Using the jam location data, the relative percentage spacing of the jams were estimated and found that it decreases down the furrow for all the cases. Hence, the number of residuejams increases down the furrow to a maximum at the tail end. Further, the number of jams is greater for NT followed by CC and ST. The infiltration and runoff data of these treatments were used for studying the effect of residuejams on these parameters. The water samples collected from these plots were analyzed for DOC concentration. Initially, DOC was high and then decreased with time in all cases. However, DOC was greater for NT followed by CC and ST. This may be due to more jams in NT followed by CC and ST. Hence, residuejams apparently affect DOC.