• Precipitation is measured with raingages.

• A raingage is an instrument that captures precipitation and measures its accumulated volume during a certain period.

• The precipitation depth is the accumulated volume divided by the collection area of the gage.

• Raingages can be
  
  
  • nonrecording,

  • recording.

• In a nonrecording raingage, rain is caught in the collector and funneled to a measuring tube, of area equal to 1/10^th of that of the collection element.

• Recording gages can be of three types:
  
  
  1. tipping bucket,

  2. weighing mechanism, and

  3. a float chamber (Page 95).

• The weighing raingage is featured in Page 96.

• The water collected by a raingage is only a small sample.

• A series of raingages constitutes the raingage network.

• Sampling errors increase with rainfall depth.

• Sampling errors decrease with network density, storm duration, and catchment area.

• The greater the duration and the catchment area, the more the temporal and spatial averaging, decreasing sampling errors.

• Error variability in precipitation is likely to be less than the error variability in model calibration.

• Therefore, care should be focused on the calibration rather than on the precipitation.

• Precipitation using telemetry is featured in Page 98.

• Radar systems are a potentially powerful tool for measuring the temporal and spatial variability of precipitation.

• Calibration is necessary when using radar systems to predict precipitation.

3.2  SNOWPACK MEASUREMENTS

• Snowpack measurements are expressed in terms of water equivalent, i.e., the depth of water that is obtained after melting a certain depth of snowpack.

• Water-equivalent data are useful in water-yield forecasts.

• Snowpack must be measured at several points.

• The snowboard is placed on the ground to permit the accumulation of snow over it.

• An inverted raingage cylinder is used to isolate a core of the new snow.

• Replacing the snowboard ready to receive fresh snow allows accumulated total snowfall to be known.

• Density is the volume of melt to the initial volume of the sample.

• Water equivalent of the snowpack can be determined from depth measurements by using known densities of snow.

• The Mount Rose sampler is commonly used in the United States.  

   

• Snow courses are selected with the objective of obtaining representative data from a given area.

• Snow courses are positioned so that they are representative not only of snowfall but also of snowmelt.

• Five snow-course samples points are adequate for well positioned snow courses with a minimum of irregularities caused by drifting or wind erosion.  

   

• Catchment water equivalent is based on point values from several snow courses.

• When snowcourses are distributed equally throughout the range of elevations, an arithmetic average of point values usually provides a satisfactory value of catchment water equivalent.

• Refinements weigh the data in proportion to its area of coverage.  

   

• Elevation is important in converting point measurements into catchment water equivalent.

• Snow courses tend to be concentrated at higher elevations, and therefore an arithmetic average is not appropriate.

• A snow chart is a plot showing the variation of water equivalent with elevation.

• This chart is used together with the catchment's hypsometric curve (area-elevation curve).

• The catchment's elevation difference is divided into several equal increments.

• For each increment of elevation, a subarea is obtained from the hypsometric curve.

• For each increment of elevation, a water equivalent is obtained from the snow chart.

• A catchment water equivalent is obtained by weighing the individual water equivalents in proportion to their respective subareas.

• Evaporation pans measure evaporation in the field.

• The pan evaporation measurement differs from the actual evaporation due to local conditions in the pan: installation and exposure.

• The pan measurement is greater than the actual evaporation in the lake or reservoir.

• The NWS Class A pan is the most widely used in the United States (Fig. 3-3).

• The NWS Class A pan coefficients are shown in Table 2-6.

• Little is known about the spatial variability of evaporation, but it appears that it is not as large as that of precipitation.

• A density of one station per 5000 km² is sufficient.  

   

  Evapotranspirometers

• Evapotranspirometers measure potential evapotranspiration.

• An evapotranspirometer consists of a central tank and at least two other watertight soil tanks.

• During one time period, the difference between the amount of water input to the soil tanks and the amount of water accumulated in the collecting can is the water lost to evapotranspiration, provided the proper allowance is made for changes in moisture storage in the soil tanks.  

   

  Lysimeters

• Lysimeters measure actual evapotranspiration.

• Actual evapotranspiration is much more difficult to measure than potential evapotranspiration.

• Actual rate is determined by climatic factors, the ability of the plant to extract water from the soil, and the speed of movement of soil moisture to the plant roots.

• A properly constructed lysimeter must be representative of the surrounding area.

• Size of lysimeter tank is usually much greater than evapotranspiration tank.

• The larger the tank, the lesser the influence of edge effects.

• Set of monolith lysimeters at Coshocton, Ohio are 2.4 m deep and 3.1 m in diameter.

• Lysimeters at San Dimas National Forest are the largest in the United States; 26 with capacities of 64 tons each.

• Infiltration rates vary greatly, in both time and space.

• Infiltration rates are determined either by the use of infltrometers or by the analysis of rainfall-runoff data from natural catchments.  

   

  Infiltrometers

• Infiltrometers are instruments designed to measure the rate at which water is absorbed by the soil surface enclosed within a small, clearly defined area.

• There are two types:
  
  
  1. flooding,

  2. sprinkler.

• A flooding infiltrometer consists of two concentric metal rings that are inserted a distance of 2 to 5 cm into the ground.

• The same water head is maintained in the annular space between the rings.

• Insertion of ring disturbes the soil.

• Flooding condition is not representative of actual field conditions.

• Infiltration estimate with flooding infiltrometer is high.

• Many tests are required to assess spatial variability.

• The sprinkler infiltration applies a simulated rainfall condition to a small plot using sprinklers.

• The simulated rainfall is continued for as long as necessary to attain an equilibrium runoff condition at the plot outlet.

• Infiltration rate is calculated as the difference between the constant rainfall rate and the constant runoff rate.

• Due to spatial variability, actual field conditions are more likely to be obtained from rainfall-runoff analysis.  

   

  Infiltration rates from rainfall-runoff data

• The procedure resembles the calculation of a φ-index.

• The rainfall hyetograph is integrated to calculate the total rainfall volume.

• The runoff hydrograph is integrated to calculate the total runoff volume.

• The difference is the infiltrated volume.

• The average infiltration rate is equal to the infiltrated volume divided by the rainfall duration.

• Method is usually limited to small and midsize catchments with negligible long-term storage.

• Discharge at a given location can be evaluated by
  
  
  • measuring stage and using a rating curve to obtain discharge (indirect method),

  • measuring the area and mean velocity of the stream (direct method).

• A good rating curve is crucial to the determination by the indirect method.

• The quality of the rating is evaluated in terms of its stability and permanence.

• A stable rating remains constant in time, i.e., the effects of flow nonuniformity, unsteadiness, or erosion and sedimentation are negligible.

• A permanent rating is one that is not likely to be disturbed by human activities.

• The stage-discharge rating should be closed to single-valued.

• Either section or channel control is necessary for the rating to be single-valued.

• A section that forces critical flow provides a section control.

• A long channel of relatively uniform cross-sectional shape, constant slope, and bottom friction, provides a channel control.

• A gaging site relying on channel control requires periodic calibration to check its stability.

• Gaging station should be located far from downstream backwater effects caused by reservoirs, large river confluences, or tides.  

   

  Stage measurements

• The vertical staff gage is vertically attached to a bridge pier or a pile.

• Sectional staff gages are used to increase accesibility.

• Recording gages measures stages continuously and records them on a strip chart.

• Self-reporting gages have automatic data transmittal capabilities.  

   

  Discharge measurements

• A discharge measurement requires the determination of flow area and mean velocity for a given stage and cross section.

• Stream-gaging procedure.

• Current meters.  

   

  Chemical methods for measuring velocity

• Chemical methods are used when it is impractical to use current meters.

• Methods can be grouped into:
  
  
  • tracer

  • dilution.

• A tracer is a substance that is not normally present in the stream, and is not likely to be lost by chemical reaction with other substances.

• The velocity of the tracer plume is the velocity of the stream.

• An expedient measurement of velocity can be obtained by timing the travel of floats.

• A float travels with a speed that is about 1.2 times the mean velocity.

• In the dilution method, a concentrated solution of a substance is introduced at a constant rate at a source point.

• The flow is sampled further downstream, after complete mixing has taken place, to determine the equilibrium concentration of the mixture.

• A mass balance of flow and substance leads to the following equation:

  Cs Qs = Ce (Q + Qs)

  in which:

  • C_s = concentration of the substance solution at the source

  • C_e = equilibrium concentration of the mixture at the sampling point

  • Q_s = rate of inflow of substance solution at the source

  • Q = stream discharge

• Solving for stream discharge:

  Q = [(Cs / Ce) - 1] Qs

   

  Indirect determination of peak discharge: The slope-area method

• The high stages and swift currents that prevail during floods combine to increase the risk of accident and bodily harm.

• It is generally not possible to measure peak discharge during the passage of a flood.

• An estimate of peak discharge can be obtained by the slope-area method.

• To apply the slope-area method, the following data are required:
  
  
  • The reach length.

  • The fall, i.e., the mean change in water surface elevation through the reach.

  • The flow area, wetted perimeter, and velocity head coefficients at upstream and downstream cross sections.

  • The average value of Manning's n for the reach.

• The following guidelines are used in selecting a suitable reach:
  
  
  • High-water marks should be readily recognized.

  • The reach should be sufficiently long that the fall can be measured accurately.

  • The cross-sectional shape and channel dimensions should be relatively constant.

  • The reach should be relatively straight, although a contracting reach is preferred over an expanding reach.

  • Bridges, channel bends, waterfalls, and other features causing flow nonuniformity should be avoided.

• The accuracy of the slope-area method improves as the reach length increases.

• A suitable reach should satisfy one or more of the following criteria:
  
  
  • The ratio of reach length to hydraulic depth should be greater than 75.

  • The fall should be greater than or equal to 0.15 m.

  • The fall should be greater than either of the velocity heads computed at the upstream and downstream cross sections.

• The steps in the application of the slope-area method are the following:
  
  
  1. With known upstream and downstream flow areas and hydraulic radius, and average value of Manning n for the reach, compute the conveyances:

     Ku = (1.486/n) Au Ru2/3

     Kd = (1.486/n) Ad Rd2/3

  2. Compute the average conveyance for the reach (the geometric mean):

     K = (KuKd)1/2

  3. Assuming zero velocity-head difference, the first approximation to the energy slope is equal to the fall of water surface elevation divided by the reach length:

     Se = F / L

  4. The first approximation of the discharge is:

     Q = K Se1/2

  5. Compute the velocity heads upstream and downstream:

     hu = αu Vu2/(2g)

     hd = αd Vd2/(2g)

  6. The new energy slope is:

     Se = hf / L = [F + k(hu - hd)] / L

     where k = 1 if the reach is contracting (V_u < V_d), and k = 0.5 if the reach is expanding (V_u > V_d). The reduction in k accounts for the recovery of the flow during expansion.

  7. The new discharge is:

     Q = K Se1/2

  8. Go back to step 5 and repeat steps 5-7 until the calculated discharge (new discharge) has converged to a constant value (usually within 3-4 iterations).