Relationships of Hydro-Climate Indices (HCI) to Volumetric Flow in the Platte River Basin

In January 2012 a Dewberry team led by John Henz prepared a report entitled, “On the Predictive Value of Hydroclimate Indices to Water Supply – A Technical White Paper” for the Platte River Restoration and Implementation program (PRRIP). It dealt with the potential value of using hydro-climate indices (HCI) during the first half of the Water Year (WY) for anticipating periods of very low stream flow volume in June on the Platte River system. During the period of 1947 – 2011 the Platte River system experienced four multi-year low flow periods as identified by a comparison of June stream flow discharge. A repeatable pattern developed in three hydro-climate indices (HCI) during these periods. The Pacific Decadal Oscillation (PDO), Multi-variate ENSO Index (MEI) and the Atlantic Multi-Decadal Oscillation were all in a negative phase when the multi-year low flow periods occurred in the Platte Basin. However it was also clear that additional work was needed to develop a predictive tool backbone.

In March 2014 Dewberry completed a project entitled, Relationships of Hydro-Climate Indices (HCI) to Volumetric Flow in North Platte River Basin for the Colorado Water Conservation Board and the PPRIP. The Platte River basin acts as a major water source for Colorado, Nebraska and Wyoming. In Colorado the South Platte River and its tributaries provides water supplies for the heavily populated Front Range region that stretches from Fort Collins to the Denver metro area and extensive agricultural areas in northeastern Colorado. The Colorado Water Conservation Board (CWCB) plays an active role in the Platte Basin during both drought and flood periods.

CWCB administers flood plain regulation and flood insurance programs and also acts as a focal point for Colorado's Drought Mitigation and Response Plan. Advanced knowledge of periods of flood or drought and their impacts on water supply is an active and continuing interest of the CWCB. Figure 1 shows the Platte River system in Colorado, Wyoming and Nebraska.

An important corollary to this study is that similar relationships are very likely to exist on the South Platte River basin and other major Colorado river basins. Given the critical nature of spring runoff volume to river managers this technique offers a very positive outcome for evaluation compared to other techniques that wait until the April decision window. This work reflects the next step in the process by evaluating the utility of HCI to provide a predictive estimate of the winter snowpack and the spring runoff on the South Platte River basin. The results of this project will be presented along with the results from the North Platte study.

The database included observations of following hydro-climate indices (HCI) for the period of 1947 to 2012:

• Multivariate ENSO Index (MEI); • Pacific Decadal Oscillation (PDO); • North Atlantic Oscillation (NAO); • Southern Oscillation Index (SOI); • Atlantic Multi-decadal Oscillation (AMO); • Arctic Oscillation (AO); • Nino 3.4; • Madden-Julian Oscillation (MJO).

The study consisted of four evaluation tasks to define the sensitivity of winter and spring snowpack conditions to the subsequent April to September runoff peak and volumes. The first task will be to create a database of the HCI, 700-500mb wind direction, basin snowpack and precipitation for the period of October – April of 1947 to 2011, the period of evaluation, for the North Platte basin and the South Platte basin at elevations above 7,000 feet.

Dewberry evaluated the sensitivity of the winter season (October–April) precipitation and snowpack in the mountainous (>7,000 feet) portions of the South Platte Basins to climate regimes as defined by the prevailing 700mb (~10,000ft) -500mb (~18,000ft) wind direction/speed and temperatures and the climate regime as defined by hydro-climate indices (HCI). The HCI to be evaluated in the study are the MEI, PDO, SOI, AMO, NAO, AO and Nino 3.4 indices.

The second task evaluated the sensitivity of seasonal precipitation, snowpack and snow water equivalent for the South Platte Basin to the July-Sep, Aug-Oct, Oct-Mar and other 3-6 month HCI values, individually and concurrently, and the 700-500mb prevailing wind direction, speed and temperature, by month and season. The third task evaluated the sensitivity of seasonal precipitation, snowpack and snow water equivalent for the South Platte Basin to the Oct-Dec and Oct-Mar HCI's, individually and concurrently, and the 700-500mb prevailing wind direction, by month and season. In doing these tasks care was taken to recognize the differing responses of each Basin to the HCI, the wind direction/speed and the temperature of the cloud layer relative to snowpack, SWE and precipitation development. A key factor will be the relationship of these parameters to the runoff timing of the peak discharge and the runoff volume.

The fourth task focused on identifying the reliability of the HCI, in groups or individually, to identify low flow periods on the North and South Platte Rivers as early as possible during the Water Year. The evaluations noted the responses by flow contributions of the North Basin by different HCI, both individually and concurrently, in an effort to determine if reliable early WY projections can be made. Every effort will be made in the sensitivity tests to determine the reliability at the start of the WY in October through the early April projections. The intent will be to provide a scientifically sound appraisal of the use of HCI as an effective flow estimation tool for the PRRIP. A schematic of the process is shown in the lower figure.

The studies showed that a clear and strong relationship exists between hydro-climate indices and the spring runoff volume on the North Platte River basin. The resulting mid-January forecast of the spring runoff volume in terms of above, below and average values is about 73 percent or almost three out of four years a correct outcome is predicted. A proxy technique to estimate the spring flow volume was developed for use until a more detailed model can be developed and will be discussed. Results from the South Platte River study will be presented at the conference.