(a) Schedule of flow quantities. The environmental
flow standards adopted by this subchapter constitute a schedule of
flow quantities made up of subsistence flow, base flow, and high flow
pulses. Environmental flow standards are established at 21 separate
measurement locations in §298.330 of this title (relating to
Environmental Flow Standards).
(b) Subsistence flow. The applicable subsistence flow
standard varies depending on the seasons as described in §298.305
of this title (relating to Definitions). For a water right holder
to which an environmental flow standard applies, at a measurement
point that applies to the water right, the water right holder may
not store or divert water under severe hydrologic conditions, unless
the flow at the measurement point is above the applicable subsistence
flow standard for that point. During severe hydrologic conditions,
if the flow at the measurement point is above the subsistence flow
standard but below the applicable dry condition base flow standard,
then the water right holder may divert or store water according to
its permit, subject to senior and superior water rights, as long as
the flow at the measurement point does not fall below the applicable
subsistence flow standard.
(c) Base flow. The applicable base flow level varies
depending on the seasons as described in §298.305 of this title
and the hydrologic condition described in §298.320 of this title
(relating to Calculation of Hydrologic Conditions). For a water right
holder to which an environmental flow standard applies, at a measurement
point that applies to the water right, the water right holder is subject
to the base flow standard for the hydrologic condition prevailing
at that time. For all measurement points except those on the Colorado
River below Lake Travis, the water right will be subject to one of
the following: a dry, an average, or a wet base flow standard. For
all measurement points on the Colorado River below Lake Travis, the
water right will be subject to either a dry or an average base flow
standard. For all measurement points, the dry base flow standard applies
during severe hydrologic conditions. For a water right holder to which
an environmental flow standard applies, at a measurement point that
applies to the water right, when the flow at the applicable measurement
point is above the applicable base flow standard, but below any applicable
high flow pulse levels, the water right holder may store or divert
water according to its permit, subject to senior and superior water
rights, as long as the flow at the applicable measurement point does
not fall below the applicable base flow standard for that hydrologic
condition.
(d) High flow pulses. High flow pulses are relatively
short-duration, high flows within the watercourse that occur during
or immediately following a storm event.
(1) For measurement points on the Colorado River below
Lake Travis, two pulses per season, one pulse per 18 months, and one
pulse per two years are to be passed (i.e., no storage or diversion
by an applicable water right holder), if applicable, and as described
in §298.330 of this title, if streamflows are above the applicable
subsistence or base flow standard, and if the applicable high flow
pulse trigger level is met at the applicable measurement point. The
water right holder shall not divert or store water until the daily
average flow at the applicable measurement point equals at least the
high flow pulse trigger level on consecutive days equaling the duration
time except during times that streamflow at the applicable measurement
point exceeds the applicable high flow pulse trigger level.
(2) For measurement points on the Colorado River above
Lake Travis, tributaries of the Colorado River, and in the Lavaca
River Basin and the Colorado-Lavaca and Lavaca-Guadalupe Coastal Basins,
one or two pulses per season and one pulse per year are to be passed
(i.e., no storage or diversion by an applicable water right holder),
if applicable, and as described in §298.330 of this title, if
streamflows are above the applicable subsistence or base flow standard,
and if the applicable high flow pulse trigger level is met at the
applicable measurement point. The water right holder shall not divert
or store water until either the applicable volume amount has passed
the applicable measurement point or the duration time has passed since
the high flow pulse trigger level occurred except during times that
streamflow at the applicable measurement point exceeds the applicable
high flow pulse trigger level.
(3) If the applicable high flow pulse trigger level
does not occur in a season, then the water right holder need not stop
storing or diverting water to produce a high flow pulse. The water
right holder is not required to release water lawfully stored to produce
a high flow pulse.
(4) Each season is independent of the preceding and
subsequent seasons with respect to high flow pulse frequency.
(5) High flow pulses are independent of the hydrologic
conditions set out in §298.320 of this title.
(6) If a high flow pulse requirement for a one-per-season
pulse is satisfied for a particular season, one of the two-per-season
pulse requirements is also considered to be satisfied. When a pulse
flow requirement for an annual pulse is satisfied in a particular
season, the one-per-season pulse requirement and one of the two-per-season
pulse requirements are also considered to be satisfied.
(e) Stored water. A water right owner that has stored
water in accordance with the terms and conditions of its water right,
including any applicable environmental flow requirement in effect
at the time the water was stored, may divert, release, or use this
water, even if the applicable environmental flow requirement is not
met at the time of the subsequent diversion, release, or use of that
stored water.
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