Programme Poster session 2
Natural flows determination in gauged hydrological basins (Part II)
effect of time scale
Author(s): Humberto Silva-Hidalgo, Ignacio R. Martín-Domínguez, María Teresa Alarcón-Herrera,
Centro de Investigación en Materiales Avanzados, S.C.
Cervantes 120. Complejo Industrial Chihuahua
31109 Chihuahua, Chih. México.
Keyword(s): Irrigation return flows, Natural flows, Water availability, Water balance, Hydrological assessment
Session: Poster session 2
In this work the effect of the time scale on the determination of natural
flows in gauged hydrological basins is analyzed, using available hydrometric historic records and the mass
The upper - middle basin of the Florido river (Rio Florido), in the northern Mexican State
of Chihuahua, was used to perform this study. The basin was divided into the upper sub-basin (U-SB), with no
infrastructure for water use, and the middle sub-basin (M-SB), where the well-infrastructured Irrigation District 103
is located. The mean annual natural flow for both sub-basins was estimated for the annual and monthly time scales,
for the period spanning from 1982 to 2002. There are no return flows in U-SB, while in M-SB flows are generated
by the irrigation district.
By correcting the annual hydrogram of mean monthly natural flows, it was estimated that
30% of the irrigation water volume returned to the river during the analysis period, as is discussed in a parallel study
(Silva-Hidalgo et al., 2008). The estimated natural flow was 116.8 Mm3 for the U-SB at the annual scale, and
119.9 Mm3 at the monthly scale. These results differ in only 2.65%, which shows that the time scale had very low
impact. The mean annual natural flow in M-SB, calculated using a return flow of 30% as determined before, results
in 59.87Mm3 at the monthly scale and of 49.6 Mm3 at the annual scale. These results are inconsistent, since they
differ 20.7% from each other. This suggests that the time scale is indeed very important in basins where water is used
The inconsistency of the results obtained at different time scales can be explained if it is observed
that, during the calculations for both sub-basins, negative natural flows were obtained during several years for some
of the dry months between February and June. A negative natural flow does not have physical meaning, but can be
related to water losses along the river, or to water extractions larger than reported.
The detection and correction
of the anomaly, if it shows up, can only be achieved when a monthly or shorter time scale is used. The correction is
performed by setting all negative calculated values to zero; the obtained results are more reliable than those obtained
using the annual time scale. It is concluded that for basins with irrigation infrastructure, the monthly time scale should
be used for natural flow determinations. The use of an annual scale is only advised for preliminary evaluations.