WHAT IS SOIL? soil is a complex system of organic and inorganic (mineral) compounds Soil properties depend on formation process particle size climate

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1 Lecture 5, Soil water and infiltration WHAT IS SOIL? soil is a complex system of organic and inorganic (mineral) compounds Soil properties depend on formation process particle size climate THE IMPORTANCE OF SOIL MOISTURE IN CATCHMENT HYDROLOGY From a systems point of view: which output signal (Q) you get from a given input signal (P) depends very much on the system (catchment) Input (P) System Output (Q) (Catchment) Imagine the extreme cases of rain falling on impervious ( ogenomtränglig ) land (e.g. asphalt) or sandy, dry soil The response will be totally different For Swedish conditions the strong influence of soil moisture conditions on storm response is well established A dry soil with large soil moisture deficit will act as a sponge ( svamp ) and can absorb all the rain without any water coming through as runoff. The soil in the unsaturated zone also acts a natural filter with respect to water quality. Water movement in the unsaturated zone is mainly vertical, in the saturated zone it is mainly horizontal. CONCEPTS / DESCRIPTION The unsaturated zone is a very complex 3 phase system containing solid particles, liquid water as well as vapour and air. In the ground water zone only 2 phases are present (solids, and liquid). Water content is normally given as % by volume (volumetric), θ [m 3 m -3 ], sometimes as % by weight (gravimetric) θ [g g -1 ]. With all pores completely filled with water, you have saturated conditions θ s. If you let a saturated sample drain until drainage stops you have field capacity θ fc. When there is so little water that a plant can not suck any water you have the wilting point θ wp The difference θ fc - θ wp denotes available water ( växttillgänligt vatten ). The difference θ s - θ fc denotes gravitational water ( dränerbart vatten ) or specific yield. Effective porosity refers to the same volume.

2 pf CURVES Implicit in the description above is the fact that an unsaturated soil can hold water against gravity. This means that suction has to be applied in order to extract water Accordingly there is a relationship between (vacuum) pressure and water content. This is often given graphically and denoted the pf curve. pf stands for 10 log(- pressure in cm head of water), c.f. ph. Each type of soil has its own characteristic pf curve. The pf curve exhibits hysteric behaviour (i.e. different curves for drying and wetting processes). Infiltration Infiltration is the downward motion of water in the uppermost soil layer. Continued motion further down is percolation During rainfall, surface runoff will occur if the infiltration capacity is less than rain intensity (the deeper soil layer may still be unsaturated) - Hortonian surface runoff Another (more common for Swedish conditions) mechanism for surface (or near surface) runoff is due to saturation from underneath. Saturated surface runoff. Infiltration is governed by the moisture gradient and soil characteristics. Models The simplest model for infiltration is the Φ index method. Here it is assumed that a given fraction of each storm will infiltrate with a constant velocity. A more realistic approach is due to Horton f cap = f c + (f o - f c ) e -kt f cap = infiltration capacity, as a function of time f o = infiltration capacity at beginning of storm f c = (final) constant infiltration rate under saturated conditions The Horton model presumes that water is available on the surface (ponding). If precipitation rate (P int ) is less than f cap then actual infiltration is P int Many more models for infiltration exist.

3 Lecture 6, Groundwater Groundwater INTRODUCTION Ground water is an important part of the water cycle Globally, a large proportion of the freshwater exists as groundwater Ground water is often used for water supply In Sweden 50 % of municipal water is ground water (half of which is from artificial recharge) Groundwater conditions are also important from a construction angle (land subsidence etc.) Groundwater terms Ground water exists in layers or aquifers If the aquifer is overlain by an (almost) impermeable layer, then we have a confined ( sluten ) aquifer. If there is no such layer on top the aquifer is an unconfined ( öppen ) aquifer. The water table is at the level where we have atmospheric pressure in the unconfined aquifer. In the confined aquifer we have the piezometric surface which corresponds to the hydraulic head in the aquifer. When the water table in an unconfined aquifer is lowered by H m, the corresponding extracted water is S y times H. S y = specific yield (or effective porosity) In general terms the amount of water corresponding to a lowering of the hydraulic head (by ΔH) is S times ΔH, where S = storage coefficient. For unconfined aquifers S = S y while for confined aquifers S is a much smaller number. In this case the amount of water released has to do with the elasticity of the soil matrix. GROUND WATER MOVEMENT Basic relationship is due to Darcy (experiments in 1856) V = -K dh/ds V = Q/A (Darcy velocity), A = total cross-sectional area K = hydraulic conductivity H = hydraulic head (level + pressure head) Actual velocity V = V d /n eff since water is only flowing in the pores K depends on both the solid matrix and the fluid (for water the viscosity is important, and therefore also temperature.) Maximum velocity = 2*V The minus sign indicates that the flow is positive in the direction where dh/ds is negative

4 GROUNDWATER FLOW The groundwater flow can be calculated using the continuity equation and the Darcy law. In this course, only stationary conditions will be treated. WELLS We assume that we have homogeneous, isotropic, and stationary conditions. In a confined aquifer: h 0 -h w = Q/(2πKb)ln(r 0 /r w ) r = radial coordinate from the center of the well index 0 = original conditions index w = well b = height of the aquifer, K*b is sometimes called the transmissivity, T. The values of r 0 can be found in the literature. In an unconfined aquifer: h 0 2 -h w 2 = Q/(πK)ln(r 0 /r w ) COMMENTS Instead of r 0 and r w any value of r can be used to calculate the corresponding h. The drawdown from several wells can be added to calculate the total drawdown.

5 Lecture 7, Runoff and hydrograph theory Q - ROLE IN HYDROLOGY River runoff is the most obvious part of the hydrologic cycle. It has also been the classical study object for the civil engineer (in connection with dams, dikes, bridges etc.) Rivers are of great use for society. Many cities were founded on the brinks of a river. (Damascus, London, Paris etc.) From a water quality point of view, the river integrates all the effects in the drainage basin. It is an indicator of the environmental status of the whole catchment. PHYSICAL ASPECTS. The river hydrograph can be seen as the response of the system (Catchment) to the input in terms of precipitation. As we discussed earlier, the response depends on the state of the system (non-linearity) According to a classical concept, runoff is created when infiltration capacity was exceeded by the rain intensity. Later this was substituted by a more complex view. We can say that water flowing in the river can have arrived there via three paths; either as overland flow, ground water flow or as interflow (i.e. flow in the unsaturated zone). In reality the individual water particle might well use all three paths for parts of the way. RAINFALL RUNOFF MODELLING In many instances it is necessary to be able to make prognoses of Q-values. The need may be to make short term prognoses (early flood warning systems for example) or it may be to somehow forecast values 50 years ahead in time (when designing large scale structures). Depending on the purpose, different tools are obviously needed. Many different types of models exist - from simple regression type of models to highly sophisticated models, which try to incorporate sub-models for all the hydrological processes. One way of simplifying the analysis is to separate the base flow.

6 BASE FLOW SEPARATION In Swedish rivers we normally find water flowing even long times after the last rain or snow melt input. Such base flow must exist because of the connection between ground water and river flow (unless the main source is a lake). Shortly after a rainstorm occurs, the river flow increases. If we can separate that part of the flow, which would have occurred even without rain, this is called base flow separation. The remaining part of the flow is directly linked to the rainstorm. It is called direct runoff. With a simplified view of the processes one could say that base flow is ground water while direct runoff is surface (overland) flow. In reality it is impossible to make such distinctions. However for engineering purposes, the distinction between base flow and direct runoff is useful.

7 Lecture 8, The Unit Hydrograph BASIC PRINCIPLES The unit hydrograph deals with the relationship between effective rain and direct runoff. Since all losses have been subtracted and since direct runoff is directly related to the net rain, the volumes of effective rain and direct runoff must be equal. The UH is based on a hypothetical case of 1 unit (1 mm) of rain falling uniformly over the whole catchment during a time interval T. The TUH gives the response to that rain. The basic assumptions of the UH are: the time base of the hydrograph remains the same irrespective of the rain intensity the UH is linear (proportionality and superposition applies) the UH is time invariant There are many objections to the concept of UH, but it has proven to be a useful tool. TECHNIQUES First the UH is found using known data. Then the UH can be used to predict the hydrograph for any given hyetograph (i.e. rain intensity as a function of time) Finding UH from a simple rain storm: IF the total effective rain volume is 5.4 mm m- divide all direct discharge values by 5.4 to get the UH Using UH for multiple storm when storm duration is nt (n is integer): Use proportionality and principle of superposition to get the total hydrograph. If storm duration is NOT nt. You can get UH for any storm duration through the method of the S -hydrograph. To find UH from multiple storm: use the proportionality and principle of superposition and find UH from the resulting system of equations. SYNTHETIC UH If there is no data for the specific catchment, it is sometimes possible to construct a synthetic UH. This is usually based on some sort of empirical functions which correlate UH with basic morphologic data of the catchment together with knowledge of catchments in the region.