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Key Question: How much water? For the sake of further discussion, we will make the (unsupported) assumption that the future annual water supply at Ft. Quitman will mimic 80% the 1976-97 average: ~140,000 acre feet, approximately half of which will be supplied by (theoretically manageable) return flows and half by unmanageable) precipitation events (typically occurring between August and October)*. On a gross balance, the 140 kaf average supply equates to ~385 acre feet/day or an annual mean streamflow of 193 cfs. The "manageable" portion would be ~193 acre feet/day and "manageable" mean streamflow would be 97 cfs.
Supplying the target maximum of 5000 cfs for 5 days (with four ramping days) would require that 65,000 af be released in a 9 day period. Thus, by using the maximum target flow, the design flood would exhaust practically the entire average year's manageable supply of 70 kaf. Supplying the target minimum of 3000 cfs for three days (with three ramping days) would require that 18,000 af be released in a 6 day period.
By using this amount, we reserve 52 kaf of the average year's "manageable" supply, or 76 cfs mean streamflow. If we use the average between the maximum and minimum requirements, it suggests that 36,500 kaf be devoted to supplying the streamflow targets, as above. (We will base the rest of this discussion on this figure.)
Logistical challenges: We have noted that the gross annual supply is delivered in an erratic and unreliable pattern, reflecting management responding primarily to the fluctuating water demands of irrigation districts and cities. Assuming the socio-political challenges to shaping a more natural hydrograph can be overcome, the logistical hurdles remain. Both the ability to "call" Project water from the reservoir (in much the same way as offstream users do at present) and the ability to prevent its depletion will be required.
Although it was outside the scope of this report to analytically model the water release requirements of a managed streamflow regime transported over 200 miles through the six diversion features, we will hypothesize that a pool of environmental water amounting to ~ 36.5 kaf might serve the purpose.
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*[Note that one day at one cfs=1.98 acre feet.]
Project water rights of 35-40,000 acre feet will need to be acquired if we hope to create the proposed environmental benefits.
Conclusion- Accomplishing the objectives of the "Forgotten River Restoration" project, restoring an appropriate degree of hydrologic and ecological function in the Rio Grande below Caballo Reservoir, will depend upon restorationists' success in negotiating streamflows at times and in amounts that will actually permit such functions.
The assistance of water engineers will be required to achieve an unprecedented level of water management flexibility, and the participation of agricultural and municipal interests will be necessary to administer new streamflow regimes.
Some amount of water would have to be acquired and storage earmarked for the project. We hypothesize that this may be as little as 35 kaf (<5% of depletable supply).
An effort to correlate releases with flow targets must be made. Conveyance losses for a range of conditions should be estimated. The whole hydraulic system (including diversions and return flows) should be modeled. The fact that there are relatively few diversion headings will simplify the effort to balance management of the river for both diversionary and instream uses. There may be side-benefits to this new management scheme, including more reliably delivering EPPSB's drinking water in a treatable condition, achieving water quality standards, even in drought and maintaining river channels (not to mention enhancing ecosystems) above the Forgotten River section.
Though a novel idea, the Rio Grande could "own" the amount of water required for semi-annual, channel-shaping design "floods". In addition, such a storage pool of water "owned" by the environment, could provide the management flexibility needed to shape other limbs of the hydrograph and permit the augmentation of streamflow in times of drought, to prevent "zeroing out the river".
Flood Capacity: Assuming that a maximum design flow of 5000 cfs would occur at a time when a maximum 2000 cfs of irrigation water would be in channel, what would be the effect of the resultant 7,000 cfs peak on lives and property in the Mesilla and El Paso/Juarez valleys?
A flood event of ~12,000 cfs in 1958 had no significant impact on existing structures, levees and bridges (IBWC, 1975). A ~4500 cfs event occurred in 1995, with minor flooding in the low water crossing near Ft. Selden and at Country Club Dr., near El Paso (personal observation). Although minor impacts from a 7000 cfs event are possible, potential flood impacts do not represent a significant deterrent to implementation of this proposal.
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