Site description Results and discussion

T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 207 Table 1 Meterological variables measured at NOAA Energy Flux Monitoring Sites along with model number and manufacturer of instrumentation used Meteorological variable Manufacturer Model number Air temperature and RH Vaisala, Helsinki, Finland 50Y Net radiation Radiation and Energy Balance Systems REBS, Seattle, WA, USA Q ∗ 7 Global radiation LI-COR, Lincoln, NE, USA LI-200 SB Precipitation Texas Instruments, Dallas, TX, USA – Wetness NOAA, Oak Ridge, TN USA – Soil heat flux REBS, Seattle, WA, USA – PAR LI-COR, Lincoln, NE, USA LI-190 SB Atmospheric pressure Vaisala, Helsinki, Finland PTB101B Surface temperature Everest, Fullerton, CA, USA 4000A Soil temperature NOAA, Oak Ridge, TN, USA – Soil moisture Vitel, Chantilly VA, USA Hydra and direction, air temperature and relative humidity, precipitation, net radiation, incoming global radiation, incoming and reflected photosynthetically active ra- diation PAR, barometric pressure, ground heat flux, surface wetness, and soil temperatures at six depths: 2, 4, 8, 16, 32, and 64 cm. A soil moisture sensor Vitel was installed before the 1997 summer season. This probe, which measures the dielectric constant of the soil, water, and air matrix, was placed in the mid- dle of a 10 cm soil layer. The soil moisture was then determined using the methodology outlined by Wang and Schmugge 1980. The surface temperature was measured with an infrared temperature sensor. These meteorological sensors Table 1 are sampled every 2 s with a datalogger and multiplexor CR21X, Campbell Scientific, Logan, UT, and averages are computed ev- ery 30 min, coincident with the eddy covariance data. 2.1. Data acquisition A laptop computer was configured to perform three operations simultaneously. The priority task is to re- ceive data from the sonic anemometer, comprising the components of the wind vector, the speed of sound from which the virtual temperature can be derived and the digitized H 2 O and CO 2 signals from the IRGA. For its second task, the computer retrieves the stan- dard meteorological data from the CR21X datalogger every 30 min and appends the data to an existing file. In background the third task, a terminate and stay resident TSR communications program is used to retrieve these data from the laptop computer via a cellular phone about once every 2 days. The en- tire system is powered by a bank of nine 12 VDC deep cycle batteries that are charged by eight solar panels. The system uses approximately 3 A at 12 V, continuously.

3. Site description

3.1. Little Washita Watershed, Oklahoma The instrumentation is located within the Little Washita Watershed, a tributary of the Washita River in southwest Oklahoma. Located in the southern part of the Great Plains of the United States Fig. 1, this area has a climate classification as moist and sub- humid. Annual rainfall is about 75 cm. Summers are typically long, hot and dry with an average daily high temperature in July of 34 ◦ C. The tower 34 ◦ 58 ′ N, 97 ◦ 77 ′ W was placed about one quarter mile north of State Road 19 within a grazed pasture. Pasture surrounds the 3 m tower in all sectors providing a minimum fetch of 200 m. The soil consists of a clay loam with a bulk density of nearly 1.6 gcm 3 . During the growing season, the leaf area index was estimated to range from 2 to 3 roughly in a 200 m radial foot- print surrounding the tower. The landcover is mixture of grasses and weeds with a canopy height rang- ing from 30 to 60 cm. The predominant grass, little bluestem Gramineae Schizachyrium scoparium, is a long-lived native bunchgrass often found with big bluestem grass, but is more drought resistant. 208 T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 Fig. 1. Location of the NOAA flux tower site in Southern Great Plains of the United States in southwestern Oklahoma.

4. Results and discussion

The results presented and discussed here are from data collected during the summer periods beginning 1 June and ending 31 August for the 4 years 1995–1998 hereafter denoted as Y95, Y96, Y97, and Y98. Data recovery rates for the summer periods Y96, Y97, and Y98 were 85, 99, and 94, respectively. However, dur- ing the first year of operation Y95, several periods during the summer had gaps in the eddy covariance data resulting is a data recovery rate of 65. Gap pe- riods were typically 2–3 days. The longest gap period in 1995 7 days began day 192. Water and carbon fluxes during the gap periods for all years during the day were therefore estimated by first computing ratios of water and carbon fluxes to global radiation for peri- ods adjacent to the gap periods, then using these ratios with the corresponding measurements of global radi- ation during the gap period to estimate the fluxes. The utility of these procedures was demonstrated by Brut- saert and Sugita 1992 during the first ISLSCP field experiment FIFE. For filling in gaps during night- time periods, water fluxes were assumed to be zero and a mean CO 2 flux of 0.2 mg CO 2 m 2 s was as- sumed. This nighttime carbon flux rate was obtained by averaging nighttime fluxes for the entire summer period. 4.1. Precipitation and evapotranspiration rates The total accumulated precipitation for the summer seasons ranged from 65 mm in Y98 to over 400 mm in Y96 Fig. 2. Except for Y98, the rainfall was fairly evenly distributed throughout the summer season. In T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 209 Fig. 2. Accumulated precipitation mm in years 1995–1998, for summer season day 150–240. 1998 was a drought year. Y98, no measurable rainfall was recorded for over 50 days beginning around day 165. These highly wet and dry years provide a unique opportunity to examine up- per and lower limits of total accumulated evapotran- sipiration ET and associated carbon fluxes during the summer season. It is during this 90 day period of the year that ET comprises roughly 50 of the annual wa- ter loss, and the associated daily carbon accumulation rates are highest. At the beginning of the summer period day 155, the daily water loss from evaporative processes aver- age of 10 days for all years was about 3 mm per day Fig. 3. By day 165, 15 days into the summer pe- riod, accumulated rainfall for Y95 and Y96 increased Fig. 3. Seasonal cycle of 10-day average daily evapotranspiration at the Little Washita Watershed for years 1995–1998. 1998 was a drought year. Fig. 4. Seasonal cycle of 10-day average ratio of actual evaporation to equilibrium evaporation at the Little Washita Watershed for years 1995–1998. 1998 was a drought year. to 150 mm while only about 20 mm was received in Y97 and Y98. The daily ET rates for Y95, Y96, and Y97 remained near 3 mm per day till day 200. There was virtually no additional precipitation from day 150 to 200 for Y98 and the 10-day average daily ET rates had progressively dropped to 1 mm per day. Equilib- rium evaporation was calculated as LE eq = s s + γ R n − G 2 where s is the slope of the saturation vapor pressure curve and γ the psychometric constant. LELE eq at the beginning of the summer season Fig. 4 for all years was comparable to earlier observations by Kus- tas et al. 1996 within the Washita Watershed. Except for Y98, all years maintained a LELE eq ratio of at least 0.7 through day 200. With little rain and available soil moisture in Y98, LELE eq progressively dropped from 0.65 to a minimum of 0.2 around day 205. After day 205, the LELE eq ratio for Y98 increased slightly in response to a few minor precipitation events. For the remaining years, LELE eq decreased slightly to 0.6 from day 200 to 240, although there was enough pre- cipitation to balance most of the evaporative losses for Y95 and Y96. Accumulated rainfall for Y97 from day 150 to 205 was only 80 mm and LELE eq decreased to just below 0.5. Although Y97 had only 40 mm more precipitation than Y98 in the early summer, ET rates did not de- crease until day 200. Accumulated precipitation from day 1 to 150, just before the summer period for Y97 210 T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 Fig. 5. Seasonal cycle of soil moisture upper 10 cm zone, volu- metric water content for 1997 and 1998. 1998 was a drought year. and Y98 was nearly equal at 400 mm. However, lit- tle precipitation 20 mm was recorded between day 115 15 April and day 150 30 May for Y98 while 210 mm was observed for Y97 for the same time pe- riod, providing more available stored water for evap- otranspiration. The seasonal trends in soil moisture at 10 cm for Y97 and Y98 also depict the frequency and impact of the precipitation events Fig. 5. For Y97, the volumetric water content θ v in g H 2 Ocm 3 was above 0.20 more than 40 of the time. In Y98, θ v was above 0.20 for only 3 of the time. The 10-day average air temperature at 2 m for all years peaked around day 210 but was highest in Y98, exceeding the other years by nearly 3 ◦ C Fig. 6. Fig. 6. Seasonal cycle of 10-day average 2 m air temperature at the Little Washita Watershed for years 1995–1998. 1998 was a drought year. Fig. 7. Seasonal cycle of 10-day average 64 cm soil temperature at the Little Washita Watershed for years 1995–1998. 1998 was a drought year. Higher air temperatures during Y98 were reflected in the higher soil temperatures observed at the 64 cm depth which reached 25 ◦ C, even though at the begin- ning of the summer season, the soil temperatures for Y95, Y96, and Y98 were nearly identical Fig. 7. The maximum observed surface temperature, as measured from a downward looking infrared sensor, showed similar trends with seasonal maximums around day 210 Fig. 8. The higher surface temperatures are consistent with the lower observed evaporation rates Fig. 3. However, the differences between years is striking with Y98 maximum surface temperatures that were 12 ◦ C above year Y96. Maximum surface Fig. 8. Seasonal cycle of maximum observed surface or “skin” temperature in a 10-day period at the Little Washita Watershed for years 1995–1998. 1998 was a drought year. T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 211 Fig. 9. Seasonal cycle of 10-day average daily net carbon fixation for the summertime period during 1995–1998. 1998 was a drought year. temperatures for Y97, the second warmest summer, were 4 ◦ C below the maximum observed surface tem- peratures in Y98. Visual observations in mid-July 1998 confirmed that little of the vegetation was green. Most of the grasses and weeds were brown from lack of precipitation. The seasonal variation in the daytime and night- time carbon fluxes varied considerably from year to year. The largest daily average net ecosystem ex- change NEE rate carbon fixation observed for all four summer seasons was about −3 g Cm 2 per day, with the largest fixation rates occurring in the early part of the summer period Fig. 9. Although Y96 was the wettest of the four summers, daytime carbon fixation rates were not the highest since there was a much higher frequency of cloudy days and CO 2 fixation rates were light-limited. For Y98, the range ecosystem was always a source of carbon for the day, with NEE rates ranging from 0 daily uptake offset by nighttime losses to nearly 4 g Cm 2 per day by the end of the summer period. Nighttime respira- tion losses were computed by summing the fluxes from 18:00 LST to 06:00 LST the next morning with daytime fluxes summed for the remaining 12 h. The warm moist soils of Y96 also produced the largest nighttime fluxes with losses over 2.5 g Cm 2 per day Fig. 10. During the driest year Y98, maximum nighttime losses were just above 1 g Cm 2 per day in the early summer and dropped to 0.5 g Cm 2 per day by day 200, with slightly higher rates after a few minor precipitation events Fig. 10. Fig. 10. Seasonal cycle of 10-day average daily nighttime carbon fluxes for the summertime period during 1995–1998. 1998 was a drought year. 4.2. The surface energy balance and carbon fluxes Using all data from the summer of 1998, closure of the energy balance is well within the combined uncer- tainty of the independent measurements of H, LE, R n and G, with a slope of 0.97 and offset of −16 Wm 2 Fig. 11. Ensembles of the measured diurnal components of the surface energy balance R n , H, LE, and G for a non-stressed year Y96 during mid-summer are Fig. 11. A comparison of net radiation versus the sum of sensible, latent and ground heat flux for all of 1998. 212 T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 Fig. 12. Typical diurnal cycle of the components of the surface energy balance and CO 2 flux at a grassland site in the Little Washita Watershed for non-stressed conditions. shown in Fig. 12. Although these ensembles were derived by averaging 3 days in which the atmospheric conditions were similar, the results are typical of non-stressed periods. Midday peaks of net radiation were near 650 Wm 2 , with nearly 50 of the net radi- ation partitioned into LE about 300 Wm 2 , 35 into sensible heat flux about 250 Wm 2 and 15 into ground heat flux 100 Wm 2 . The evaporative fraction LER n − G ≈ 0.46 is nearly constant and showed no dependence on vapor pressure deficit, similar to several sites in the Washita Watershed reported by Kustas et al. 1996. The evaporative fraction is lower than what was observed by Verma et al. 1992 over a similar C 4 grass ecosystem during mid-summer; conversely the sensible heat fraction H R n ≈ 0.38 is larger than the fractions observed by Verma et al. 1992. This in part could be attributed to active graz- ing that occurred throughout the summer, whereas the measurements of Verma et al. 1992 were made over ungrazed prairie. Over the diurnal cycle for this non-stressed period, LE slightly lagged behind net radiation, with H peaking earlier in the day and G peaking later around 14:00 LST. H peaked before noon LST while maximum G was around 14:00 LST, and actually exceeded H by 15:00 LST. The cycle of LE was, however, in phase and proportional with the available energy R n − G , with no dependence on vapor pressure deficit. The associated CO 2 flux was in phase with both LE and net radiation with midday up- take rates peaking near −0.75 mg CO 2 m 2 s. Night- time respiration losses were smaller in magnitude but were relatively constant 0.25 mg CO 2 m 2 s. For water stressed periods in Y98, surface en- ergy balance and carbon fluxes were very different Fig. 13. The pattern of net radiation was similar to the net radiation in non-stressed years but the midday peak was about 50–70 Wm 2 less than that for the non-stressed years. The difference can be reconciled with the increased outgoing longwave radiation during the stressed years proportional to the fourth power of the surface temperatures, which were much higher in Y98 Fig. 7. The sensible heat flux comprised the largest fraction of R n about 65, with midday val- ues that peaked near 400 Wm 2 . The second largest term was G, which accounted for nearly 25 of R n with midday values of 180 Wm 2 . LE comprised the remaining 10 of R n with midday fluxes near Fig. 13. Typical diurnal cycle of the components of the surface energy balance and CO 2 flux at a grassland site in the Little Washita Watershed for drought conditions in year 1998. T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214 213 Table 2 Seasonal day 150–240 total precipitation, evaporation, net carbon ecosystem exchange, and average ratio of evaporation to equilib- rium evaporation Year Precipitation mm Evaporation mm NEE g Cm 2 LELE eq 1995 307 273 − 196 0.65 1996 432 262 − 41 0.66 1997 190 224 − 118 0.59 1998 67 145 155 0.39 75 Wm 2 . Soil moisture values in the upper 10 cm of the soil profile remained near 0.10. With little active vegetation, the land surface was a source of CO 2 to the atmosphere with midday emis- sion rates near 0.25 mgm 2 s. Nighttime emission rates were much smaller 0.05 mgm 2 s with soil moisture levels near 0.1 Fig. 5. The minimum car- bon flux appears to occur just after sunset. During this transition period, surface layer turbulence is usu- ally suppressed as the surface quickly begins to cool. Consequently, air-surface exchange rates are often spurious and are often relatively low. For the entire summer season, total evaporation for non-drought years ranged from 224 to 273 mm Table 1. For Y98, the drought year, total evaporative loss for the summer was only 145 mm, roughly 60 of the average losses for the other years. Daily evap- oration rates for the non-drought years ≈3 mm per day are comparable to those found by Dugas et al. 1999 for native prairie in Texas. The amount of car- bon dioxide fixed each season was much more vari- able than the evaporation rates during the non-drought years, ranging from 41 to 196 g Cm 2 . The lack of precipitation in Y98 resulted in little to no carbon fix- ation from photosynthesis. This resulted in a net loss of carbon from the land surface to the atmosphere of 155 gm 2 for the summer season Table 2.

5. Conclusions