Lecture7 vegetation, landuse and runoff generation (1)

VFU, 23-September-2014 Watershed morphometry and discharge Lecture #7 Vegetation, landuse, and runoff generation Principles of Watershed management • What is the primary difference between these two basins? • How much potential hydrograph differed Dr Bui Xuan Dung- Department of Environment Management Drainage Density • The drainage density is a measure of the total length of well defined channels that drain the watershed (sometimes measured as the blue lines representing the streams on a topographic map) • Drainage density is defined as the sum of the lengths of all of the channels (km or miles) divided by the total watershed area (km2 or ft2) This ratio can be determined from topographical maps Drainage Density • Drainage density affects the response of the watershed to rainfall • High densities usually allow fast runoff removal Therefore, greater peaks and hydrographs with shorter durations are expected for watersheds with higher drainage densities • The effect of drainage density on runoff volume is associated with the time during which the runoff remains in the watershed • Low densities allow for long residence times; therefore, abstraction mechanisms have more time to remove water • Drainage density can be increased by urbanization Because of the faster response facilitated by high densities, the frequency distribution of streamflow approximately follows that of precipitation events Effects of land use on hydrograph shape Urbanization effects Vegetation Management and water yield • What are the real and anticipated effects of vegetation removal on the amount and distribution of precipitation? • What degree can water yields be manipulated by altering vegetation yield? • Can vegetation be manipulated to complement water resources management objectives? • What extent of seasonal stream flow patterns altered by changing vegetation cover? Forest Types • • • Stands - group of trees - can be from a half acre to several hundred acres Old-growth forest - forest which have never been cut Second-growth forest - cut and regrown Forest Management • Clear-Cutting - cutting all trees in a stand at one time • Shelterwood cutting – cutting dead/undesirable trees - Always have young trees Approaches, cont • Seed-tree Cutting - removes all but a few with good genetics and high seed production which will be used to promote regeneration • Selective Cutting - Individual trees are cut Thinning of small, poorly formed trees, or trees of specific sizes Landuse activity and flooding • Removal vegetation decrease interception loss • Less evapotranspiration may affect high base flow • Reducing infiltration capacity due to soil compaction increase runoff • Increasing the soil erosion decreases capacity of channel for flow Road hydrology Effects of forest roads on storm flow generation and sediment transport in mountainous catchments evapotranspiration evapotranspiration precipitation precipitation throughfall stemflow overland flow roads subsurface flow subsurface flow Does ISSF-derived erosion intercepted and solute export need to subsurface flow (ISSF) be considered? If so, road design needs to Hortonian be better planned in overland flow (HOF) relation with soil depth and subsurface flow pathway 五味 高志 Effects of forest road on flow generation and soil erosion （Hillslope scale) 林道が水・土砂流出に与える影響 （斜面スケール） Bedrock 林道のり面からの水 岩盤 Cutslope seepage Hortonian overland flow …………………………………………………… …………………………………………………………… …………………………………………………………… 地表面流 Subsurface flow 地下水流・土壌中水流 林道面からの排水 Drainage discharge 45N 135E Cutslope interception Hortonian overland flow 40N 130E 35N Stream Experimental plot Forest road plot Forest road plots 等高線 water shut plate 林道面 Convex shape gutter pipe Planar shape water gauge Road runoff monitoring V-notch weirs Concave shape 50 25 50 50 尾根 y = 0.3x R² = 0.5 0 0 50 50 100 総降雨量 (mm) 150 100 Rainfall (mm) 150 2011.5.28 - 30 Total rainfall : 62.2 mm Max 1hour rainfall: mm 2011.10.5 - Total rainfall: 64.4 mm Max 1hour rainfall: 7.6 mm kr-1 kr-2 kr-3 kr-1 kr-2 kr-3 60 17:00 19:30 22:00 00:30 03:00 05:30 08:00 10:30 13:00 15:30 18:00 20:30 23:00 09:00 10:30 12:00 13:30 15:00 16:30 18:00 19:30 21:00 22:30 00:00 01:30 03:00 04:30 06:00 谷 y = 0.5x R² = 0.6 2.5 1.5 0.5 120 Overland flow (mm) 100 平行 y = 0.5x R² = 0.7 Runoff ratio （%） kr-1 kr-2 kr-3 線形 (kr-1) 線形 (kr-2) 線形 (kr-3) 25 100 地表流量 (mm) Overland flow(mm) 50 150 Rainfall (mm/30min) 150 Effects of roads on runoff: Intercepting subsurface flow (SSF) Road intercepted runoff ratio （%） kr-1 kr-2 kr-3 (5.5) Cutslope Interception of SSF Soil Travelway API = n = 12, 17, 12 < API < 0.1 n = 5, 7, API > 0.1 n = 7, 9, Bedrock/ Restrictive Layer Interception of SSF is greatest when cutslope height > soil depth Primary Controls on Road Surface Erosion Amount of road surface runoff, which depends on: - Precipitation; - Road surface area; Hydraulics of overland flow, which depends on: - Amount of runoff; Road segment slope (energy); Road surface area x segment slope (A*S) often used to predict road surface erosion; Erodibility of the road surface = complex function of: - Soil texture; Surface cover (vegetation, rocks, litter); Pattern of flow on the road surface (e.g., rilling); Time since construction or maintenance; Age of road; Amount and type of traffic Effects of harvest operation Road and skid trail Utilization of machine Overland, Quick flow Base flow  Jones and Grant, Forest harvesting with 2-3% of the Soil1996: disturbance area in road increased peak flow from 50 toInstallation 100% of skid trail  Guillemette et al 2005: Increases in peak flow were attributed to the hydrological connectivity of overland flow via skid trails and roads Definition of hydrological connectivity 12 True or false? Road, skid trail runoff Hydrological connection No hydrological connection • The forest lower air temperature inside and above it? • Forest increase the abundance and frequency of precipitation? • The destruction of forest affect the climate? • In hill and mountain country, forests conserve water for streamflow? • Forest road affect high runoff? Stream channel 10 True and false? Difficulties for evaluation of effect of forest removal • Forest prevent erosion? • Forest regulate the flow of spring? • The total discharge the large rivers depends on forest • Forests tend to equalize throughout year and by making the low stage higher and the high stages lower • Forest cannot prevent floods production by exceptional precipitation, but they can mitigate their destructiveness Overview of forested area View within forest 11 Background Forest plantation in Japan Area (ha) Area（×1000 ha） 4e+7 40000 All land in Japan 3e+7 30000 All forest 2e+7 20000 Plantation forest 1e+7 10000 Ratio of plantation：44% Cypress plantation：26% 1970 1980 1990 2000 林業センサス累計統計書より Year vs Total Year vs Platation forest Forest plantation and runoff generation Infiltration capacity of dense Japanese cypress forest with sparse understory vegetation tends to be low Hortonian overland flow and related soil erosion is one of the major concerns in the forest management of headwater catchments in Japan Outline of Mie Site M2 M７ M5 No understory vegetation Natural Forest rain Infiltration Plantation M4 M6 M2 M4 Managed Hinoki Dense Hinoki with fern cove Bare soil surface M3 M8 Low infiltration Deciduous forest M1: 4.9ha M5 Kyoto Subsurface and groundwater flow Overland flow Mie 12 Monitoring Hortonian Overland Flow in Multiple Scales Field Infiltration Measurement Large plot 300 FIRmax [mm/h] Infiltration capacity (mm /hr) Small plot with a stem 200 Small plot 100 0.5 x 2.0 m plot scale x 25 m hillslope scale + Cover class Catchment scale Observed overland flow in the small plots Plot1(M2) Runoff coefficient： 0.28 mm/5 Plot3 (M5) Changes in hydrological processes due to thinning After thinning Before thinning Decrease interception loss Runoff coefficient： 0.41 Runoff coefficient: 0.28 Plot4 (M8） Increase throughfall 12/4/2004 12:00 12/5/2004 0:00 12/5/2004 12:00 Infiltration Overland flow Runoff •Significant overland flow occurred from the beginning of the storm event •Japanese cypress forest without understory vegetation had more runoff Increase base flow? 13 ... forest Forest plantation and runoff generation Infiltration capacity of dense Japanese cypress forest with sparse understory vegetation tends to be low Hortonian overland flow and related soil erosion... will be used to promote regeneration • Selective Cutting - Individual trees are cut Thinning of small, poorly formed trees, or trees of specific sizes Landuse activity and flooding • Removal vegetation... to Hortonian be better planned in overland flow (HOF) relation with soil depth and subsurface flow pathway 五味 高志 Effects of forest road on flow generation and soil erosion （Hillslope scale) 林道が水・土砂流出に与える影響