Irrigation Hydrology Class Lecture Notes :-
2. Chapter 1: Introduction 1.1 General introduction Hydrology is derived from two words: hydro and logos. ‘Hydro’ means water and ‘logos’ means study. Simply, Hydrology is defined as the study of water. Hydrology is the science which deals with the origin, distribution and circulation of water in different forms in land phases and atmosphere. Broad definition of Hydrology given by US National Research Council Hydrology is the science that treats the waters of the Earth, their occurrence, circulation and distribution, their chemical and physical properties, and their reaction with the environment, including their relations to living things. Interdisciplinary subject: As an earth science, Hydrology is connected to several subjects. These include: Meteorology: for understanding precipitation and evaporation process Soil science: for understanding infiltration Geology: for understanding groundwater flow Geomorphology: for understanding surface runoff Hydraulics: for understanding stream flow Physics, Chemistry, Biology, Math, statistics: to formulate and understand the subject Division of hydrology 1. Scientific hydrology: deals with hydrological processes from the view point of natural processes 2. Applied or engineering hydrology: deals with engineering applications of hydrology Scope of hydrology 1. Estimation of water resources 2. Study of processes like precipitation, evaporation, infiltration and runoff and their interaction 3. Study of problems like floods and droughts 4. Understanding the properties of water in nature Things to be considered for planning and design of water resources projects a. Maximum flows which are expected to occur at a place b. Minimum flows which can occur during any dry period c. Minimum reservoir capacity to be fixed to meet all water demands from a multipurpose reservoir d. Possible regulation of floods at the downstream reaches once a hydraulic structure is erected e. Possible supply of water from a river to meet demands for water resources projects f. Environmental impacts of a hydraulic structure g. Study of groundwater potential and its use Watershed/catchment/drainage basin
3. Watershed is the area of land draining into a stream at a given location. Divide is a line which separates catchment from its neighbouring catchments. For delineating basin, we need topographic map. The map shows changes in elevation by using contour lines. Features of contour Uphill: contour with higher elevation Hill: circular contour, ridge: highest point Saddle: mountain pass Valley: V or U shaped with the point of the V/U being the upstream end Close together contours: steep slope Widely spaced contour: level ground Basin delineation procedure on topo map Mark the outlet point Mark the highest point around the river Start from the outlet and draw line perpendicular to the contours in such a way that the line passes from the highest point (ridge) Continue to the opposite side of the watercourse, finally ending to the outlet. Finding area of watershed/basin Method 1: Use planimeter around the boundary Method 2: Trace the basin and count area manually Method 3: Use of GIS package 1.2 Applications of Hydrology in Engineering Correct assessment of flows for hydropower, irrigation, drainage and water supply projects. Determination of maximum expected flow at dam, reservoir, spillway, bridges, culverts and city drainage system. Determination of minimum reservoir capacity sufficient to meet the hydropower, irrigation and water supply demands. Estimation of the total volume of water that may be available from a drainage basin over a long period Flood control: statistical analysis of probable frequency of floods, estimation of design flood, and flood forecasting. computation of water surface profile for various rates of flow for navigation Control of erosion to minimize sedimentation of reservoirs. Reduction of stream pollution 1.3 Hydrological cycle The endless circulation of water between the earth and its atmosphere is called hydrological cycle. Hydrological cycle is the most fundamental principle of hydrology. The cycle extends its scope from 15 km up into the atmosphere from the earth’s surface to about 1km below the earth’s crust through a maze of paths. It is fueled by solar energy and driven by gravity force.