HYDROLOGY Lecture 5 GROUNDWATER Assoc.Prof. dr.tarkan erdik

Groundwater is 7500 times the water in streams Groundwater is 7500 times the water in streams. Naturally stored, free of smells and flavors. Groundwater hydrology (Geohydrology) Nearly 30% of the water in streams comes from groundwater. In dry periods, water requirement is met from the groundwater by wells. At present, 40% of the water is provided from the goundwater globally.

Regions of groundwater Unsaturated zone in the pores air + water Saturated zone in the pores only water The upper surface of saturated zone is water table. Along this surface the water pressure in the pores equals atmosperic pressure. In between, there is capillary fringe where water rises by capillary stresses.

Water infiltrating into the soil first enters the unsaturated zone, where air and water exists Water percolating deeper reaches the saturated zone. At water table (WT), pressure is atmospheric. Right above water table, there is capillary fringe Groundwater has impervious layer as the lower boundary. Streams that are above the WT feed the groundwater (influent) Streams that are below the WT are fed by the groundwater (effluent)

Variation of pressure of water in various zones

Unsaturated zone 1. In unsaturated zone, water and air exist together. 2. 3- Water in this zone cannot be withdrawn by wells. 3-This zone is important for plants and drainage problems. 4-In marshes, there is no unsaturated zone so water table reaches the soil surface.

What is pelicular (molecular) water? It is the water which is attached to the grain by molecular adhesion. Finer the grain, higher the pelicular water. When the downward motion of water by gravity ends The amount of water that cannot be removed from the soil even by plants It is rather diffucult to measure the soil moisture content. In the laboratory, the soil sample is weighed, and then dried and weighed again. The difference corresponds to soil moisture. Field capacity Higroscopic water Wilting point

Saturated Zone The pores are completely filled with water. This water can be transmitted from one point to another. Water in the pores can be withdrawn. Such a formation is called an aquifer meaning «layer carrying water» Such formations have large enough porosity, and pores of large enough size.

1.Unconfined (free surface) aquifers 1- The upper limit is the water table, along which the saturated zone is in contact with the unsaturated zone. 2-Atmospheric pressure exists at the water table because there is air in the pores of the unsaturated zone. 3-The flow in unconfined aquifer is similiar to free surface flow in an open channel.

2.Confined (pressurized, artesian) aquifers 1- This type of aquifers are confined from above by an impervious layer. This type of aquifers have no free surface in touch wtih atmospheric pressure. 2-The flow in these aquifers is similar to the pressured flow in pipes. 3-The percentage of water in an aquifer is equal to the porosity of the aquifer because all the pores are filled with water. 4-Porosity is volume of pores / total volume

Large porosity does not mean that much water can be extracted from the aquifer. Why? Because the water below wilting point cannot be extracted.

Specific yield: the ratio of the volume of water that can be extracted to the total volume is specific yield. Specific retention: the ratio of the volume of water that cannot be extracted to the total volume is specific retention. Please define porosity in terms of specific yield and specific retention Porosity=Specific yield + Specific retention

Feeding of Groundwater 1-After precipitation when the unsaturated zone reaches field capacity, by percolation. 2- By influent streams which are above water table and by lakes through seepage. 3-By seepage from irrigation channels. 4-By water rising though faults. 5-By water vapor condensing on soil, not significant.

Losses for groundwater 1-Transpiration when the capillary zone reaches the roots of plants 2-Evaporation from the soil when the capillary zone approaches the soil surface 3-When the water table intersects the soil surface a) Surface storage and evaporation b) Discharging into a stream c) The groundwater reaches to the surface (spring) 4-Pumping and artesian outflow by wells (human intervention)