Mapping ET at 1km resolution for Nebraska

Decadal mean annual recharge estimates in the Sand HillsJozsef Szilagyiresearch hydrologistSchool of Natural Resources, UNL

ra = ln(z2/z1) / u*ku*=ku200 / [ln(200/z0m)]after Allen et al. (2007)

Why recharge matters?-- Current and future well-being of agriculture in Nebraska depends on thewide availability of groundwater for irrigation

-- Groundwater that eventually ends up in the air (purposefully via irrigation) must be balanced by recharge for long-term sustainability, so recharge is the ultimate control variable in the system

-- Any regional groundwater quantity modeling needs recharge rates

-- Any groundwater quality modeling also needs recharge rates

Background on CREMAPET cools the surface very effectively land surface temperature (Ts) must be related to ET rate

From observations (included in SEBAL, METRIC models): near surface air temperature gradient (dzTa) is directly proportional to Ts

If there is a suitable spatial scale over which surface properties do not change significantly net energy at the surface and aerodynamic resistance (ra) are near constant sensible heat flux (dzTa / ra ) is directly proportional to Ts ET is directly proportional to Ts

The MODIS scaleThe 1km spatial scale of MODIS Ts data (from 2000 on) is ideal: surface albedo typically changes minimally among the cells for NE (171.2%) net energy at the surface can be near constant for flat or rolling terrain

is large enough to have various cover types within most of the cells ra may become near constant for time periods of days or longer (under neutral conditions ra relates to the logarithm of the roughness height)Slide 4

is small enough to give good resolution at the watershed scale

Transformation of Ts into ETDefiniton of two anchor points for the linear transformation:

-- Mean Ts () of a region plus mean ET (E) from the Complementary Relationship (CR) of evaporation by WREVAP (1985) of Morton

-- Ts () of the coldest surface area which cools itself at the maximum rate of ET (Ew) which derives from the Priestley-Taylor (1972) equation since the 1km scale is the effective lower scale for P-T (Brutsaert, 1982)

Application of the transformation equationThe Ts ET transformation is done on a monthly basis for altogether 8 regions in Nebraska

The monthly time-step eliminates cloud contamination of the MODIS pixels

It is the typical time-step of hydrologic models

Input data: Ts (MODIS), Ta, Td, Rs (from PRISM & GEWEX)

Mean annual ET (mm), 2000-2009

(39)(31.5)(23.5)(16)

Mean annual ET / P, 2000-2009

Check out the eastern outlines of the Sand Hills!

(8)(6)(4)(2)

Verification of CREMAP results: -- two riparian forest locations (near Gothenburg and Odessa) -- three Sand Hills locations at the Gudmundsen Ranch -- one agricultural location near Mead

Three sites: (1) wet meadow; (2) dry meadow; (3) dunal upland

(20)(18.5)

Gothenburg site

The Odessa site

(27.6)(25.2)(26)(24.5)

The Mead site

ETmeas = 642 mm/yrETest = 608 mm/yr(25.2)(24)

CREMAP ET

(20)(10)

CREMAP ET

(20)(10)

CREMAP ET

(20)(10)

(15.7)(19.7)(23.6)(27.5)Mean annual (2000-2009)precipitation in the Sand Hills

Mean: 533 mm/yr (21)

Mean annual (2000-2009) ET in the Sand Hillsmm/yr(15.7)(23.6)(19.7)(27.5)(31.5)DismalM. LoupN. LoupGoose Cr.CalamusSnakeNiobraraBirdwood Cr.The CREMAP ET rates are adjusted for the 7.6% overestimation Mean: 460 mm/yr (18)

Mean annual (2000-2009) net recharge in the Sand Hills(4)(8)(12)RechargeDischargeMean: 7350 mm/yr (2.871.97)The error-boundscome from an assumed 5% error in P, ET

Mean annual (2000-2009) net recharge to precipitation ratio (1 ET/P) in the Sand HillsRechargeDischargeMean: 149 %

The effect of irrigation to recharge rates:

Center pivots in the Sand Hills

No induced net rechargeeffect for center pivots:

it is the oppositeWhile center pivots mean only reduced recharge rates in the eastern part of SH,their boosted ET surpass precipitation ratesin the western part (net recharge is negative)

Effect of afforestation (Ponderosa pine) on ET National Forest near Halsey, NE

Verification of the method within the Sand Hills:

-- Clover (1972), Chen & Chen (2004) report a recharge to P ratio of 13% for the Middle-Loup and Dismal Rivers, together-- From USGS discharge records it becomes 16% and 11%, resp.

-- The current method gives: 99% and 129%

-- From USGS discharge records we have 13% for the North-Loup-- The current method gives: 149%

The average recharge rate for the 3 adjacent watersheds is 13% fromdischarge values compared with our 129%, or 6545 mm/yr (2.561.77)

References:

Brutsaert, W., 1982. Evaporation into the Atmosphere, D. Reidel, Dordrecht, the Netherlands.

Chen, X., Chen, X., 2004. Simulating the effects of reduced precipitation on groundwater and streamflow in the Nebraska Sand Hills, J. Water. Res. Assoc., 40(2), 419-430.

Clover, R. E., 1972. Deep percolation in a Sand Hills area, J. Water. Res. Assoc., 8(2), 399-400.

Morton, F., Ricard, F., Fogarasi, F., 1985. Operational estimates of areal evapotranspiration and lake evaporation program WREVAP. National Hydrological Research Institute Paper #24, Ottawa, Ontario, Canada.

Priestley, C., Taylor, R., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters, Monthly Weather Rev. 100, 81-92.

Szilagyi, J., Kovacs, A., 2010. A calibration-free evapotranspiration mapping (CREMAP) technique for spatially-distributed regional-scale hydrologic modeling, J. Hydrol. Hydromech., in press.

Szilagyi, J., Kovacs, A., 2010. Complementary-relationship-based evapotranspiration mapping (CREMAP) technique for Hungary, Periodica Polytechnica, 54(2), 95-100.

Special thanks to Dave Billesbach, Shashi Verma, Andy Suyker, Suat Irmak from UNL, and the USGS office in Lincoln for sharing their ET data!