Modern Climate and Hydrology

1
Modern Climate and Hydrology

• Lester McKee
• Alicia Gilbreath

2
Hypothesis

• Land use change through time has increased runoff
  volume and peak flow in the lower reaches of
  tributaries and in the mainstem Napa River due to
  net influence of reservoir storage (runoff from
  30 of the land area) and farm dam storage,
  changes in vegetation cover (in some cases
  increases in forest/woodland cover and in most
  cases a shift from open space to agriculture),
  soil compaction, hardening of channels, increased
  drainage density and connection of tributaries.

3
Rainfall through time
4
Runoff is variable
5
Area Discharge relationEvidence to modify H1?
6
Rainfall-Discharge relation is variable Evidence
to modify H1?
7
Basin-wide rainfall and runoff(1892-2005)

• MAP 514 mm (20.24 in)
• MAR 69 mm (or 13 MAP)

8
Return interval of floods Evidence to modify H1?
9
Lag-to-peak analysis

• NOAA hourly rainfall data obtained for Saint
  Helena for 1950-present
• Time of peak discharge for the peak annual floods
  obtained from USGS
• Analysis showing spurious results (6-18 hours lag
  with no relation to flood magnitude

10
Low Flow

• Lines roughly parallel until 85th percentile
• Beyond this, it appears
• Tulucay Creek and Napa River at Saint Helena
  remain better supported by ground water supply
• Milliken Creek and the lower mainstem Napa River
  lesser supported?

11
Low-flow Evidence to modify H1?
POTW dry season discharge prohibited
Drought
12
Return interval of low-flow
13
Rainfall Low-flow relation(Napa nr Napa)
14
What are the data telling us?

• No change in runoff at the annual scale
• Smaller floods appear to have gotten larger
• No evidence that suggest a reduction in low flow
• Only at rainfall days

15
Modified hypothesis

• Land use change through time has increased runoff
  volume and peak flow in the lower reaches of
  tributaries and in the mainstem Napa River during
  small floods only and has had no impact on dry
  season flows
• Floods
• Tile drains
• Increased connection
• Low-flow
• Tile drains
• Incision