Relations for estimating unit-hydrograph parameters in New Mexico

Data collected from 20 U.S. Geological Survey streamflow-gaging
stations, most of which were operated in New Mexico between
about 1969 and 1977, were used to define hydrograph characteristics
for small New Mexico streams. Drainage areas for the gaging stations
ranged from 0.23 to 18.2 square miles. Observed values for
the hydrograph characteristics were determined for 87 of the most
significant rainfall-runoff events at these gaging stations
and were used to define regional regression relations with basin
characteristics. Regional relations defined lag time (tl), time
of concentration (tc), and time to peak (tp) as functions of stream
length and basin shape. The regional equation developed for
time of concentration for New Mexico agrees well with the Kirpich
equation developed for Tennessee. The Kirpich equation is based on
stream length and channel slope, whereas the New Mexico equation is
based on stream length and basin shape. Both equations, however,
underestimate tc when applied to larger basins where tc is greater
than about 2 hours.

The median ratio between tp and tc for the observed data was
0.66, which equals the value (0.67) recommended by the Natural
Resources Conservation Service (formerly the Soil Conservation
Service). However, the median ratio between tl and tc was only
0.42, whereas the commonly used ratio is 0.60.

A relation also was developed between unit-peak discharge (qu)
and time of concentration. The unit-peak discharge relation is
similar in slope to the Natural Resources Conservation Service
equation, but the equation developed for New Mexico in this study
produces estimates of qu that range from two to three times as
large as those estimated from the Natural Resources Conservation
Service equation.

An average value of 833 was determined for the empirical constant
Kp. A default value of 484 has been used by the Natural
Resources Conservation Service when site-specific data are not
available. The use of a lower value of Kp in calculations generally
results in a lower peak discharge. A relation between the
empirical constant Kp and average channel slope was defined in this
study. The predicted Kp values from the equation ranged from 530
to 964 for the 20 flood-hydrograph gaging stations. The standard
error of estimate for the equation is 36 percent.