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Lake Evaporation using Energy Budget Method: Walker Lake, NV - a case study In cooperation with Bureau of Reclamation. Kip K. Allander (kalland@usgs.gov) 2008 USGS National Ground-Water Workshop August 4, 2008 Lakewood, CO. Outline. Why lake evaporation is important.
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Lake Evaporation using Energy Budget Method: Walker Lake, NV - a case studyIn cooperation with Bureau of Reclamation Kip K. Allander (kalland@usgs.gov) 2008 USGS National Ground-Water Workshop August 4, 2008 Lakewood, CO
Outline • Why lake evaporation is important. • Overview of different methods. • Energy budget method. • Case study – Evaporation from Walker lake, NV. • Some useful references.
Why is evaporation from Lakes important? • Water budgets. • Evaporation is often large discharge component of lake water budget. • Water management. • Leads to better understanding of other water budget components such as ground water. • Lake water quality.
Methods • Water budget • Potential ET • Pan Evaporation • Energy budget
Energy Budget Method Qn: Net radiation Qv : Heat advection Qb : Net energy stored in lake-bed sediments -Assumed negligible in deeper lakes Qe : Energy used for evaporation Qh : Sensible heat Qw : Energy advected by evaporation Qx : Change in heat storage in lake
Energy Budget Method - continued ρ: Density of water (103 kg m-3) E : Evaporation rate (m/s) L : Latent heat of vaporization (J kg-1) B : Bowen ratio (unitless) c : Heat capacity for water (J kg-1°C-1) Te : Temperature of evaporated water, taken as lake surface temperature (T0; °C) Tb : Base temperature reference, usually 0 (°C)
Net Radiation (Qn) • Critical component that is central driver of evaporation process. • One of the easier components to measure. • Measure directly using a net radiometer.
Heat Advection (Qv) Fx : Flux of water by process. Inflows and outflows of SW, GW, Precipitation (m s-1) Tx : Temperature of water flux (°C)
Heat Advection (Qv) - Inflow • What is needed to measure Qv inflow? • Measure inflow and T of SW. • Measure or estimate GW inflow and T. • T of precipitation is wet bulb T during precip events.
Heat Advection (Qv) - Outflow • What is needed to measure Qv outflow? • Measure outflow of SW, T can be estimated from Lake T. • Measure estimate GW outflow T can be estimated from lake T. • Heat advected by evaporation is already incorporated into governing equation.
Layer 1 Layer 2 Layer 3 Layer n Heat Storage (Qx) c : Heat capacity for water (4,187 J kg-1°C-1) ρ: Density of water (103 kg m-3) t : Time of measurement period (s) di : Thickness of layer (m) Ai : Area of layer (m2); A0: Lake surface area (m2) ΔTi : Change in temperature of layer (°C)
Heat Storage (Qx) • What is needed to measure Qx? • Lake bathymetry. • Temperature profile measurements every 1 to 4 weeks.
Bowen Ratio (B) P : Atmospheric Pressure (Pa) Ca : Specific heat of air (J kg-1°C-1) T0 : Water surface temperature (°C) Tu : Temperature of air at height above lake surface (°C) e0 : Vapor pressure at lake water/air interface (Pa) eu : Vapor pressure at height above lake surface (Pa)
Lake surface temperature (T0) • Critical component of calculation. • Is also one of the easier components to measure. • Measure directly using thermistor.
Evaporation from Walker Lake • Background • Problem • Objectives • Data • Results • Summary
Walker River Basin - Background • Walker River Basin is a topographically closed basin with DA of 3,950 mi2. • Source of Walker River is in the Eastern Sierra Mountains, a humid continental climate with high annual precipitation.
Walker River Basin - Background • Supports large agricultural economy in four valleys on its journey downstream. • Terminates in Walker Lake, a terminal lake (no surface outflow) set in mid-latitude desert climate, with low annual precipitation and hot, dry summers.
Walker Lake E – Problem • Lake is in decline due to diversions of source water to upstream AG. • Salt mass is relatively constant leading to increasing TDS. • Freshwater fishery is severely stressed, entire lake ecology is threatened. • Water is needed to preserve ecology of lake, but how much?
Walker Lake E – Problem • Original estimate of E was developed using water budget method. • Potential problems with original estimate: • Assumed GW was negligible. • Stream inflow based on record a substantial distance from Walker Lake. • Uncertainty on how streamflow was converted to depth measurement.
Hydrograph for unused well 4.5 miles south of Walker Lake 42 ft decline in ground-water level (~0.3 ft/yr) Walker Lake Stage Walker Lake E – Problem
Walker Lake E - Objectives • Determine evaporation from Walker Lake. • Develop an improved water budget for Walker Lake.
Walker Lake E - Data • Platform installed Nov 2004 and operated through Nov 2006. • Water temperature profiles measured approximately every month for entire period.
Walker Lake E – Summary • Walker Lake is a terminal lake in recession which is threatening ecologic collapse. • An accurate water budget is necessary for decisions on how to save Walker Lake while minimizing impact on AG economy. • Evaporation from Walker Lake is greater than previously thought by about 25,000 ac-ft/yr. • Groundwater as a source of inflow to Walker Lake is greater than previously thought.
Open water evaporation – References Click on box to open a short, incomplete list of references related to lake evaporation energy-budget method.
