Module:�Rainfall-Runoff Analysis Using Real-World High-Frequency Data

© 2023 Manoj K. Jha, Professor, North Carolina A&T State University

Characteristics of Rainfall

Spatial and Temporal Variation

Rainfall varies greatly both in time and space:

With respect to space – Spatial Variation

With respect to time – Temporal Variation

The temporal variation may be defined as hourly, daily, monthly, seasonal variation, and annual/yearly variation.

Point Precipitation (P)

• Rainfall at a point is measured by an instrument, called rain gage.
• Rainfall data collected at a point is representative of rainfall of the surrounding area.
• It is measured as vertical depth of water in “mm” or “inches” at a point.
• Precipitation over a given area is estimated using area-weighted method over a network of several rain gages.

Hyetograph

It is a plot of the accumulated precipitation or rainfall intensity against time, plotted in chronological order. In other words, it is a graphical representation of rainfall or intensity over time for a given area.

Hydrograph

• Hydrograph at a given point in the stream network shows the relationship between discharge or streamflow (Q) over a period of time (t)

Hydrograph – Volume Calculation

Baseflow Estimation

• Baseflow is an important component of the groundwater system. It is the component of streamflow that is attributed to ground water discharge and other delayed sources such as snow melt into streams.
• Knowledge on baseflow availability is important in development of water management strategies, especially for drought conditions; establishment of relationships between aquatic organisms and their environment; estimation of small to medium water supplies; and management of salinity, water quality, and algal blooms. Additionally, baseflow maintains flow for navigation, water supply, hydroelectric power and recreational uses in reservoirs.
• Base flow displays spatial and temporal variability due to climate, land use, soils, frequency and amount of recharge, vegetation, topography, and geology
• Exact separation of the streamflow hydrograph into surface flow and groundwater flow is difficult and time consuming. There is several software developed over the years to help with the estimates of baseflow including USGS BFI method (Wolock 2003), Recursive Digital Filter method (Santhi et al. 2007), PART (Rutledge 1993), HYSEP (Sloto and Crouse 1996), etc.

Baseflow Separation Methods (Hydrograph Separation)

Baseflow Separation Techniques

Method 1: Join B and F by a straight line

Method 2:

• Extend AB further beneath the peak
• Extend FG backwards beneath the peak
• Join both extensions

Method 3:

• Extend AB to beneath the inflection C
• Join with F

Method 4:

• Extend FG backward beneath the inflection point E
• Join with B

Method 5

• Estimate N (time from the peak when runoff ends)

N = A^0.2 (N: # of days, A: drainage area in km²)

• Extend AB beneath the peak and join the hydrograph at N days after the peak

N

Rainfall-Runoff Relationship

Hyetograph vs. Hydrograph

• Hyetograph is rainfall (P) over time (t) but the hydrograph is runoff or discharge or streamflow (Q) over time (t)
• Hydrograph provides Q vs. t at a given point in a stream network whereas hyetograph represented the precipitation over a given area
• Hyetograph and hydrograph are also termed as rainfall-runoff relationship. Precipitation converts into runoff after incorporating all losses (infiltration, evaporation, consumption etc.)