Observations of the Precipitation-Evaporation Balance in Northern Florida Bay

Topical Area: Meteorology/Remote Sensing


Ned P. Smith, Harbor Branch Oceanographic Institution, Fort Pierce, Florida

DeWitt Smith, National Park Service, Everglades National Park, Homestead, Florida


The hydrology of Florida Bay is poorly understood, in spite of its fundamental importance to the ecology of the bay. The import or export of fresh water has not been estimated from measure­ments made along the western or eastern boundaries of the bay, and neither surface runoff nor groundwater moving south from the Florida Peninsula has been quantified accurately. Climato­logical data from Miami and Key West suggest that the multi-annual average rainfall should be between 146 cm (Miami) and 100 cm (Key West), but wet and dry years can raise and lower annual totals substantially. Evaporation estimates have not been made for Florida Bay, however data are available from the National Park Service Marine Monitoring Network and from a one-year Army Corps of Engineers field study. The combination of meteorological and hydrographic data from these two sources permits calculations of rainfall and evaporation, and thus preliminary estimates of the precipitation-evaporation balance of the northern part of Florida Bay.


The principal objective of this study is to compare freshwater gains and evaporative losses over diurnal and seasonal time scales. Gaps in weather records and water temperature records limit time periods suitable for calculations, but even with breaks in the time series seasonal patterns and annual accumulations can be examined. Results provide a first look at the local precipita­tion-evaporation balance that controls the hydrology and salinity of the poorly flushed interior of the bay.


Weather stations were installed by the Army Corps of Engineers on platforms over open water near Johnson Key and Butternut Key. Data were recorded as quarter-hourly averages from mid March 1996 through late April 1997. Each weather station recorded air temperature, relative humidity, atmospheric pressure and wind speed and direction. Humidity was checked in March 1997, and errors were assumed to accumulate linearly from the start of the study. The Army Corps of Engineers weather stations did not record water temperature, but National Park Service surface temperature measurements were available from Marine Monitoring Network (MMN) study sites near Johnson Key and Butternut Key.


A weather station has been in operation in Joe Bay since 1993 as part of the National Park Service MMN data collection program. Air and surface water temperature, relative humidity, atmospheric pressure, wind speed and direction are recorded quarter-hourly. Also, rainfall data have been recorded at Blackwater Sound, Duck Key, Highway Creek, Little Blackwater Sound, Little Madiera Bay, Long Sound, Trout Cove and Taylor River since 1993, and spatial averages from these eight stations are used to characterize the seasonal cycle of precipitation.


Evaporative water losses were quantified using a bulk aerodynamic formula that involves the product of the air density, a non-dimensional bulk aerodynamic coefficient, the scalar average wind speed and the specific humidity difference between the water surface and the level at which weather observations are recorded. The scalar average wind speed at a standard height of 10 m was estimated from measurements made at a height nominally 5.2 m above the bay surface.


To emphasize short-term variability, hourly evaporation is plotted as a function of time; to emphasize longer time scales, cumulative evaporation values are plotted. Over sub-seasonal time scales, the slope of the least-squares regression line quantifies the mean evaporative water loss. For ecological applications, the precipitation-evaporation (P-E) difference is of primary concern, because it is the net gain or loss of water that will lower or raise salinity. Plots of P-E are conceptually similar to water level time series that would be obtained from evaporation pan mea­surements. Diurnal variations in evaporative water loss are characterized by a Buys-Ballots aver­aging of all the 0100, 0200, etc. evaporation rates from the time series. This obscures day-to-day fluctuations in the diurnal cycle by constructing a mean diurnal variation. Harmonic analysis is used to characterize the magnitude of the diurnal cycle, as well as the times of maximum and minimum evaporation rates.


The annual cycle of evaporative water losses, as calculated from the 1993-1997 Joe Bay data base, is closely approximated by the sum of 12-month and 6-month period sine waves defined by harmonic analysis. Results indicate that an annual cycle with amplitude of 0.14 cm day-1 about a mean value of 0.37 cm d-1 explains 93% of the variance in the multi-annual monthly ave­rages. The sum of the first and second harmonics explains 95~O of the total variance and suggests that evaporative losses reach an annual maximum in early June. The multi-annual monthly means calculated from observations include a relatively broad maximum, with values of approximately 0.5 cm day-1 from April through July. The Joe Bay data base suggests some­what lower values in June, producing twin-peaked maxima in May and July. This may be a result of the relatively short weather record. Minimum evaporation rates of 0.2 cm day-1 occur in the months of December and January.


The annual cycle of precipitation, defined by the 1993-1997 monthly averages from eight study sites in northeastern Florida Bay, includes a broad minimum from November through April. Monthly mean rainfall rates are 0.15-0.2 cm day-1, except for a 0.33 cm day-l value in January. Maximum values, representing the wet season, cover the period from June through October. The August, September and October values are nearly equal at about 0.53 cm day-1. June and July appear to be anomalously high and low, respectively, with values of 0.70 and 0.33 cm day-1. The ragged appearance of the annual precipitation curve suggests that a substantially longer record is needed to average out the effects of interannual variability.


Day-to-day fluctuations in evaporation at the Joe Bay, Johnson Key and Butternut Key weather stations in have been examined from early December 1996 to early March 1997. At all three locations, the magnitude of the daily peak is highly variable. During periods of strong winds and/or low humidity, peak evaporation rates can exceed 0.05 cm h-1 at Johnson Key, 0.06 cm h-1 at Butternut Key and 0.07 cm h-1 at Joe Bay. For much of the record, however, peak values do not exceed 0.02 cm h-1 at Johnson Key and Joe Bay, and 0.01 cm h-1 at Butternut Key. During this 84-day period of time, mean evaporation rates at Joe Bay, Johnson Key and Butternut Key were 0.0151, 0.0124 and 0.0101 cm h-1, respectively. Some features appear at all three locations, such as the times of highest evaporation rates and periods of sustained evaporation. But the Butternut Key plot has a fundamentally different appearance in the sense that the diurnal varia­bility is distinctly lower. This, together with the different mean values, suggests that evaporation may have significant spatial variability at different locations across Florida Bay.


Buys-Ballots averaging reveals the mean daily cycle of evaporation and provides the input for harmonic analysis. The diurnal variation calculated from the Joe Bay data base has a relatively sharp crest and a relatively broad trough. Evaporation exceeds the daily mean value from 1100 EST to 2030 EST, or about 40% of the day. Harmonic analysis indicates that over 99% of the variance can be explained by the 24-hour and 12-hour period harmonics. Together, they trace a diurnal cycle with a minimum evaporation rate just before 0600 EST, and a maximum evapora­tive water loss at 1430 EST.


Joe Bay data from early February to late December 1995 provide the longest uninterrupted record for calculating and comparing evaporation and precipitation. Combining freshwater gains by pre­cipitation with evaporative water losses, one obtains a picture of the hydrologic signature of the wet and dry seasons. The 1995 Joe Bay data indicate that from early February through early June, evaporative water losses exceed precipitation, and the result is a net water loss. From early June through mid October, freshwater gains from precipitation exceed losses by evaporation, even though evaporative losses are highest at this time of year. The 1995 wet season ends abruptly in mid October, and for the rest of the year evaporative losses are relatively constant with little indication of any significant amount of precipitation.


In other years, seasonal imbalances of evaporation and precipitation stand out less clearly. Heavy rainfall in late February 1994 produced a net freshwater gain during what is normally the dry season. Also, except for a relatively wet June, the dry season extended into mid August. Signifi­cant amounts of rainfall during the last half of September, then again in mid November, produce a net freshwater gain for the year.


The combination of the National Park Service and Army Corps of Engineers data bases provides a useful preliminary picture of evaporative water losses from Florida Bay, and it provides num­bers for comparisons with other gains and losses of fresh water. Using the 0.371 cm d-1 (135.53 cm y-1) multi-annual mean water loss calculated from the Joe Bay weather data, and applying this value to the 2,220 km2 surface area of the bay east of the 81°05'W meridian, one obtains a total annual water loss of approximately 3.01 x 109 m3. This is equivalent to 0.343 x 106 m3 h-1, but this annual mean value will increase by as much as 40% in midsummer months, then decrease by 45% in midwinter months. Even tentative conclusions regarding the precipitation-evaporation balance in Florida Bay are difficult, because of the spatial variability, and especially the inter­annual variability in precipitation. However, the 0.346 cm day-1 mean rainfall during the 1993­1997 study period produces a precipitation minus evaporation difference of -0.025 for northern Florida Bay. Related work is needed to quantify spatial gradients in precipitation and evapora­tion, freshwater runoff and the contribution of groundwater to complete an understanding of the water budget of Florida Bay.