PRECIPITATION - Is any liquid or frozen water that forms in the atmosphere and falls to the earth All forms of water that reach the earth from the atmosphere To form it the atmosphere needs moisture, moist air undergoes lifting and resultant cooling, phase change due to condensation Forms of precipitation: 1. 2. - Drizzle – light steady rain in fine drops 0.5 mm, intensity: <1mm/hr Rain – condensed vapor, falls in drops > 0.5 mm from clouds; max 6 mm Type Light Rain Moderate Rain Heavy Rain Intense Rain Torrential Rain Intensity, mm/hr Trace to 2.5 2.5-7.5 7.5-15 15-30 >30 3. 4. 5. 6. 7. 8. 9. Glaze – freezing of drizzle or rain (contact with cold objects Sleet – frozen rain drops (cold air) Snow – ice crystals due to sublimation (water vapor to ice) Snowflakes – ice crystals fused together Hail – small lumps of ice (> 5mm diameter) Due to freezing and melting Dew – moisture condensed in small drops upon cool surfaces Frost – feathery deposit of ice formed in the ground or surface of exposed objects 10. Fog – thin cloud by condensation of atmospheric vapor 11. Mist – thin fog Classification of Precipitation 1. Thermal Convection (convective lifting) - Form of local whirling thunder storms 2. 3. 4. 5. - Warm air rises due to its low density then cools to a cauliflower shaped cloud Bursts into a thunder storm Called tornadoes when accompanied by destructive winds Conflict between two air masses (frontal lifting) 2 air masses with contrast in temperature, precipitation will occur Orographic lifting Mechanical lifting of moist air over mountain barriers Heavy precipitation on the windward side Cyclones (cyclonic precipitation) Due to lifting of moist air converging into a low-pressure belt (due to pressure differences - unequal heating of earth) Winds blow spirally inward counterclockwise in the northern hemisphere and clockwise in the southern hemisphere Tropical Cyclone (hurricane or typhoon) 300 to 1500 km (small, high wind velocity, heavy precipitation) Extra-tropical cyclone (anti-cyclone) – large diameter (3000 km), causes wide spread frontal type precipitation Rainfall Measurement Rain gauge – measures rainfall rate in a certain period of time 1. - Non-recording type Cylindrical vessel with enlarged base Symons rain gauge Measured 3 to 4 times in a day Measured by a graduated measuring glass inside the receiving bottle 2. Recording type - Automatic mechanical arrangement (clockwork, drum with graph paper, and a pencil point) a. Weighing Bucket Type Rain Gauge - Most common self-recording rain gauge - Certain weight of water makes a pen move on a chart wrapped around a clock-driven drum. b. Floating or Natural Syphon Type Rain Gauge - Has a float that rises on top of the container as level rises in the container c. Tipping Bucket Type Rain Gauge - Circular rain gauge consisting of a sharp-edged receiver and provided with a funnel inside - 0.25 mm rainfall tips the bucket and is discharged into the container EVAPORATION - Transition from liquid to gas below the temperature at which it boils Water to water vapor Commonly expressed in mm/hr or in/hr Evaporation depends on: 1. Vapor pressure at the water surface and air above Dalton's law: evaporation rate, E, controlled by two factors, the windspeed and the saturation deficit: E = u ( es-ea ) u: function of windspeed ea : current vapor pressure es: saturation pressure at that temperature 2. 3. 4. 5. Air and water temperature Windspeed Quality of water Size of the water body Evapotranspiration (Total Evaporation) (ET) - Evaporation from soil, water bodies and loss of water from plants. ET = T + E Evaporimeters: - Water-containing pans which are exposed to the atmosphere and the loss of water by evaporation measured in them at regular intervals Lake Evaporation = Cp x Pan Evaporation Cp = Pan coefficient Values of pan coefficient: Types of Pan Average Value Class A Land Pan 0.7 ISI Pan (modified 0.8 class A) Colorado Sunken 0.78 Pan USGS Floating Pan 0.8 - Range 0.6 – 0.8 0.65 – 1.10 0.75 – 0.86 0.70 – 0.82 We usually use Class A and USGS As per range (they also usually use the maximum range but to be safe, use the mean amount – av. value) Water molecules are tightly held by intermolecular forces – energy is consumed in evaporation to move the molecules away Energy = latent heat if vaporization (λ) = 2.5 MJ/kg *megajoules per kg λ = 2.501 – 0.00236T (T = temperature in Celsius) Water to vapor – water vapor pressure is low (vaporization) = evaporation Water vapor to liquid (condensation) If the rate of vaporization equal condensation, the air is saturated and evaporation stops Difference of Saturated vapor pressure (eS) and vapor pressure in the air (ea) determines the rate of evaporation **Bigger difference = more evaporation Factors that affect evaporation in open water a. Solar radiation b. Temperature of water and air (ex. Clothes) c. Difference in vapor pressure between water and the overlying air d. Wind speed across the lake The amount of water evaporated from water surface is estimated by: a. Evaporimeter b. Empirical formula c. Analytical Formula Practice Problem A reservoir with a surface area of 250 hectares had the following average values of climate parameters during a week: a. Water temp – 20°C b. Relative humidity – 40% c. Wind velocity at 1 m above ground surface – 16 km/h Estimate the average daily evaporation from the lake by using Meyer’s Formula RAINFALL-RUNOFF RELATION STORM HYDROGRAPH (FLOOD HYDROGRAPH) - A graph showing the relationship between rainwater and discharge in a river - Mainly used to observe discharge for a given storm event - Shows how storm has affected a river IMPORTANCE OF STORM HYDROGRAPH Flood control flood control structures on flood plains such as Dikes (levees) Drainage canal (divergent) For flood forecasting Concepts: Peak stream flows are a result of storm rainfall Components: AB - Baseflow recession BC - rising limb how engineers know what to forcast gives indication of how fast water is reaching the channel (represents level of water) CD - falling limp shows the river as its level flows DE - baseflow recession BASEFLOW SEPARATION METHOD A. STRAIGHT LINE METHOD - Drawing a horizontal line from the point at which the surface runoff begins to intersect with the recession limb - For ephemeral streams B. FIXED BASE METHOD - Surface runoff is assumed to end a fixed time “N” after the hydrograph peak - Base flow before surface runoff began is projected to the time of peak, and then projected on the recession limb at time “N” after peak C. VARYING/VARIABLE SLOPE METHOD - The base flow curve before surface runoff is extrapolated forward to the time of peak discharge and then the base flow curve after surface runoff ceases is extrapolated backward to the point of inflection on the recession limb. A straight line is used to connect the endpoint of the extrapolated curves Main timing aspects of the storm hydrograph: 1. Duration of rainfall excess (D): - Time from start to finish of rainfall excess 2. Lag time (tp): - Time from the center of mass of the rainfall excess to the peak of the hydrograph 3. Time of rise (TR): - Time from the start of rainfall excess to the peak of the hydrograph 4. Time of concentration (tc): - Time for a wave of water to propagate from the most distant point in the watershed to the outlet. One estimate is the time from the end of net rainfall to the inflection point of the hydrograph 5. Time base (Tb): - Total duration of the DRO (Direct Runoff) hydrograph PROBLEM SOLVING 1. Determine the DRH (direct runoff hydrograph) assuming an initial baseflow of 400cfs, the ∅-index, and the ERH from the observed rainfall and streamflow data given in the table. The watershed is 7.03 sq.mi. Unit Hydrograph States that “basin outflow resulting from 1.0 inch (1.0 mm) of direct runoff generated uniformly over the drainage area at a uniform rainfall rate during a specified period of rainfall duration.” Procedure for analysis: 1. Analyze the hydrograph and separate base flow. 2. Measure the total volume under the DRH and convert this to inches (mm) over the watershed. 3. Convert total rainfall to rainfall excess through infiltration methods, such that rainfall excess = DRO, and evaluate duration D of the rainfall excess that produced the DRO hydrograph. 4. Divide the ordinates of the DRH by the volume in inches (mm) and plot these results as the unit hydrograph Problem solving: Following are the ordinates of a storm hydrograph of a river draining a catchment area of 423 km2 due to a 6-h isolated storm. Derive the ordinates of a 6-h unit hydrograph for the catchment. t = Time (Fv) start of storm (h) Q = Runoff discharge (m3/s) t Q t Q -6 10 54 39 0 10 60 31 6 30 66 26 12 18 24 30 87.5 115.5 102.5 85 72 78 84 90 21.5 17.5 15 12.5 36 71 96 10 42 48 59 47.5 102 10