Hydrological Study of a stream catchment in Palau Pacific HYCOS - SOPAC 39 th STAR Session - Nadi 18 th October 2010

Presentation Outline Pacific HYCOS - Hydrological study on a large catchment in Palau, Island of Babeldaob Why is such a study necessary How is this undertaken What is required What are the outputs

Why is this data needed in Palau ? Pacific HYCOS - Box culverts Bridging Water availability for small scale Irrigation & food production

Pacific HYCOS - Design of water storagesDam safety (PMF), spillway design & dambreak studies (Ngerimel Dam 20 Mill gallons) Water availability and demand – weirs, intakes and treatment plants

Pacific HYCOS - Catchment health, water safety planning In stream flow requirements, base flow assessment Ridge to reef considerations, eg GBRMPA Tourism opportunities

From field data to planning/design Pacific HYCOS -

Palau’s National Hydrological Archive Pacific HYCOS - Only one data file for all of the national data

Measurement of continuous water levels Pacific HYCOS - Float & weight recorders Bubbler systems & automatic raingauges Gauge boards

Rating curve development Pacific HYCOS - Medium flows and floods Low flows A provisional curve only, no medium to high flows measured Note extrapolation based on questionable USGS old curve

Tabercheding Catchment Pacific HYCOS - Catchment area km 2 Catchment perimeter km Specific min low flow yield April litres/km 2 * Specific flood yield July m 3 /s per km 2 * * Very limited data available

Sample outputs - water level only Pacific HYCOS - USGS data HYCOS data

Sample outputs – rated (flow) data Pacific Islands Applied Geoscience Commission Level + rating = flow data, we can use this if the ratings are accurate USGS HYCOS

Min and maximum flows Pacific HYCOS - A short dataset with no calibrated flow rating cannot be used for flood analysis, however our very provisional rating indicated the maximum flow was 115 m 3 /s, but It could be 150 m3/s or 80 m3/s, what are the implications ?

Flow duration/distribution Pacific HYCOS - Important for engineers, designers and planners, how much water is available and how long is it there, especially for low flow analysis What % of time can a particular demand be supplied, Eg we may be required to meet a 95% reliability factor, whereas measured data may indicate that only 70% can be met What do we do, reduce the demand ? alternative water ?

Monthly mean flows Pacific HYCOS - Note – insufficient data to establish the mean flow, need min of 5 years

Daily mean – perhaps Mar/Apr were approaching drought ?? Pacific HYCOS -

Flow vs daily rainfall – what is the link ? Pacific HYCOS - Note – the rain gauge is at the lower end of the catchment If these were floods, might there be a relationship between rainfall & runoff ?

Water availability during a dry period in 2010 Pacific HYCOS - What have we learnt ? 7 day minimum Q of up to 38/lsec (up to 23,000 m 3 available for 7 days or average of 3,280 m 3 /day) Minimum Q of 15/lsec on 15 th April (1300 m 3 available) Would this compromise reliability of a water demand scheme ? This is based on a 18 month dataset, but it can assist in planning and project design, if we had 10 years of good continuous data ???

In closing …. Pacific HYCOS - Pacific HYCOS Project is at an end Unless ongoing substantial support is available to NHS’s sustainable data collection activities will remain compromised especially for datasets to verify and mitigate the effects of: Climate change Flood risk – DRM Plus Infrastructure design Water resource planning and management A range of water related projects “You cant manage what you don’t measure”