variations of wtd in space and time determine where and when pools of water form at the land surface , thus creating potential mosquito breeding habitats .
other meteorologic variables , as well as soil and vegetation type and antecedent conditions , must be considered if evapotranspiration , water movement within the soil column , and river runoff are to be quantified .
topography must also be constrained if the flow of water across the land surface , runoff rates , and the local convergence of water in lowlands ( surface pooling ) are to be modeled accurately .
we combined a soil column model , which simulates the vertical movement of water and heat within the soil and between the soil surface , plus vegetation and the atmosphere , with the topmodel ( topography - based hydrology model ) approach ( 1720 ) , which incorporates topographic data to track the horizontal movement of shallow groundwater from the uplands to the lowlands .
topmodel formulations permit dynamically consistent calculations of both the saturated fraction within the watershed ( partial contributing area ) and the groundwater flow that supports this area , from knowledge of the mean depth of the water table and a probability density function for soil moisture deficit derived from topographic statistics . using the model , we can produce a three - dimensional picture of soil moisture distribution within a catchment .
this approach to modeling the land surface has been validated at several catchments , ranging in scale from the red arkansas basin ( 570,000 km ) ( 21 ) to the black rock forest catchment ( 1.34 km ) ( 22 ) .
sentinel chickens were used to measure slev transmission . the annual timing and distribution of slev transmission to sentinel chickens have been strongly correlated with slev in humans ( 1 ) .
data derived from five sentinel flocks maintained in indian river county were used in this study .
map of indian river county , florida , with numbered locations of the five sentinel chicken flocks .
m.  sentinel chicken flocks were maintained by personnel from the indian river mosquito control district . from 1986 to1990 ,
flocks with six birds each were placed in the field by mid - june and removed at the end of december . in 1991 , the year after the slev epidemic , surveillance was year - round .
a 1.0-ml blood sample was drawn once a week from each bird during peak transmission periods ( july through november ) and twice a month during the rest of the year .
blood samples were assayed for hemagglutination inhibition antibodies to slev at the florida department of health and rehabilitative services , tampa branch laboratory .
individual chickens testing positive for hemagglutination inhibition antibodies were replaced with fresh sentinels , and the entire flock was replaced each spring .
we defined slev transmission intensity for each sentinel flock as the number of seropositive chickens per weekly sample .
we also defined slev transmission incidence for each flock as a categorical data set : one , if one or more chickens were slev seropositive per weekly sample ; or zero , if no chickens were seropositive .
data from all sentinel sites were also pooled , and slev transmission intensity and incidence were similarly determined .
western indian river county is dominated by citrus groves intermixed with hammock  islands  of southern live oak and cabbage palm ( 23 ) .
, at least three times per week , one 20-minute collection was made approximately 2 hours after sunrise with a portable ground aspirator along a transect at a hammock site 6.4 km southwest of vero beach ( 27 38 n , 80 27 w , see figure 1 ) .
collected mosquitoes were sorted by species , categorized by sex and gonotrophic condition , and counted .
hourly meteorologic data were assembled from national climate data center archives for vero beach , florida .
gaps in the record were filled with hourly data from national climate data center archives for melbourne and west palm beach .
solar radiation data were provided by the northeast regional climate center from analysis of the national climate data center data by using the northeast regional climate center solar energy model ( 24 ) .
geological survey national elevation dataset digital elevation model of south - central florida , using tardem version 4 routing freeware ( 25 ) . soil and
the hydrology model was run from 1984 through 1995 and provided a daily series of mean wtd for the study area .
because of the channelization and water control in south florida , the model was validated by using groundwater well measurements and surface ( canal ) water levels , provided by the st .
the partitioning of runoff and evapotranspiration matched bulk estimates taken from u.s . geological survey and st .
univariate and bivariate logistic regression was used to associate the probability of slev transmission incidence with single time lags of modeled wtd and combinations of two time lags of wtd . whole model goodness - of - fit
was measured by log - likelihood ratio and the pseudo r - squared ( uncertainty ) coefficient .
sentinel chickens were used to measure slev transmission . the annual timing and distribution of slev transmission to sentinel chickens have been strongly correlated with slev in humans ( 1 ) .
data derived from five sentinel flocks maintained in indian river county were used in this study .
map of indian river county , florida , with numbered locations of the five sentinel chicken flocks .
m.  sentinel chicken flocks were maintained by personnel from the indian river mosquito control district . from 1986 to1990 ,
flocks with six birds each were placed in the field by mid - june and removed at the end of december . in 1991 , the year after the slev epidemic , surveillance was year - round .
a 1.0-ml blood sample was drawn once a week from each bird during peak transmission periods ( july through november ) and twice a month during the rest of the year .
blood samples were assayed for hemagglutination inhibition antibodies to slev at the florida department of health and rehabilitative services , tampa branch laboratory .
individual chickens testing positive for hemagglutination inhibition antibodies were replaced with fresh sentinels , and the entire flock was replaced each spring .
we defined slev transmission intensity for each sentinel flock as the number of seropositive chickens per weekly sample .
we also defined slev transmission incidence for each flock as a categorical data set : one , if one or more chickens were slev seropositive per weekly sample ; or zero , if no chickens were seropositive .
data from all sentinel sites were also pooled , and slev transmission intensity and incidence were similarly determined .
western indian river county is dominated by citrus groves intermixed with hammock  islands  of southern live oak and cabbage palm ( 23 ) . dense ground cover makes these hammocks an excellent daytime resting site for cx .
nigripalpus in all gonotrophic stages ( 2 ) . during 1986 through 1991 , at least three times per week , one 20-minute collection was made approximately 2 hours after sunrise with a portable ground aspirator along a transect at a hammock site 6.4 km southwest of vero beach ( 27 38 n , 80 27 w , see figure 1 ) .
collected mosquitoes were sorted by species , categorized by sex and gonotrophic condition , and counted .
hourly meteorologic data were assembled from national climate data center archives for vero beach , florida .
gaps in the record were filled with hourly data from national climate data center archives for melbourne and west palm beach .
solar radiation data were provided by the northeast regional climate center from analysis of the national climate data center data by using the northeast regional climate center solar energy model ( 24 ) .
geological survey national elevation dataset digital elevation model of south - central florida , using tardem version 4 routing freeware ( 25 ) . soil and
the hydrology model was run from 1984 through 1995 and provided a daily series of mean wtd for the study area .
because of the channelization and water control in south florida , the model was validated by using groundwater well measurements and surface ( canal ) water levels , provided by the st .
the partitioning of runoff and evapotranspiration matched bulk estimates taken from u.s . geological survey and st .
univariate and bivariate logistic regression was used to associate the probability of slev transmission incidence with single time lags of modeled wtd and combinations of two time lags of wtd . whole model goodness - of - fit
was measured by log - likelihood ratio and the pseudo r - squared ( uncertainty ) coefficient .
all five sentinel flocks had slev transmission recorded during the study period ( 19861991 ) .
figure 2 provides a time series of slev transmission intensity and weekly averaged modeled wtd .
modeled wtd was lowest in 1989 and 1990 , matching a period of drought in vero beach ( based on palmer drought severity index records , data not shown ) .
three instances of slev transmission ( the late summer and early fall of 1986 , 1989 , and 1990 ) were recorded .
all three episodes occurred during a wetting period ( rising of the water table ) that followed a drought ( low wtd ) .
the two larger instances of slev transmission intensity in the vero beach area , 1989 and 1990 , were reported during the wet conditions that followed a prolonged drought .
this sequence of hydrologic conditions , antecedent and coincident with slev transmission , is similar to the scenario suggested by provost ( 10 ) .
time series of weekly seroconversion of sentinel chickens ( transmission intensity ) and weekly averages of modeled mean water table depth ( wtd ) .
univariate logistic regression was performed to explore the relationship between slev transmission incidence and time lagged modeled wtd . a range of time lags ( 029 weeks )
table 1 provides a list of the best - fit logistic regression results produced by this analysis , and figure 3 presents these results graphically .
all logistic regression models were highly statistically significant ( p<0.0001 ) ; in fact , a range of time lag values ( generally 1025 weeks ) produced statistically significant models ( p<0.001 , data not shown ) .
all five sites show the same trend : slev transmission incidence was strongly associated with low wtd 16 to 25 weeks before onset of slev transmission .
wtd , water table depth ; the probability of slev transmission incidence is represented as a function of single time lags of weekly averaged modeled wtd . whole model goodness - of - fit was assessed by log - likelihood ratio and the pseudo r - squared ( uncertainty ) coefficient .
a ) site 1 ; b ) site 2 ; c ) site 3 ; d ) site 4 ; e ) site 5 ; f ) all five sites , florida . a second
, bivariate logistic regression analysis was performed to explore the effects of both antecedent drought and coincident wetting conditions .
modeled wtd time lags of 10 to 25 weeks were paired with modeled wtd time lags of 0 , 1 , 2 , or 3 weeks and used together in bivariate analysis of slev incidence .
table 2 provides a list of the best - fit model equations resulting from this analysis .
the optimal range of fits among sites is more tightly constrained when two variables are used ( range 1217 weeks before transmission for antecedent drought , 12 weeks before transmission for coincident wetting ) .
the bivariate models are also more statistically significant than their univariate counterparts ( based on log - likelihood ratio whole model goodness - of - fit ) , and the parameter estimates of both explanatory variables are statistically significant ( p<0.01 ; p=0.068 for site 2 ) .
the probability of slev transmission incidence is represented as a function of two time lags of weekly averaged modeled water table depth ( wtd ) .
whole model goodness - of - fit and individual parameter estimates were assessed as per the univariate analysis .
figure 4 presents the bivariate model fit of slev incidence for all five sites combined .
figure 4a shows the logistic regression fit for a continuous range of modeled wtds 2 weeks before transmission and fixed values of modeled wtd 17 weeks before transmission .
this figure shows that antecedent drought conditions are necessary for slev transmission ; only with a modeled wtd of < 1.2 m 17 weeks before transmission is there any probability of slev transmission .
this probability , however , is modulated by a rise in the wtd 2 weeks before transmission .
this moderating effect is also shown in figure 4b , which fixes values of modeled wtd 2 weeks before transmission but allows the conditions 17 weeks before transmission to vary .
combined , these two explanatory variables ( modeled wtd 17 weeks before transmission and 2 weeks before transmission ) offer a strong prediction of slev transmission . best fit bivariate logistic regression model of st .
a ) plotted for a continuous range of modeled water table depths ( wtds ) 2 weeks before transmission and fixed values of modeled wtd 17 weeks before transmission ; b ) plotted for a continuous range of modeled wtds 17 weeks before transmission and fixed values of modeled wtd 2 weeks before transmission .
the results from analysis of slev transmission incidence with modeled wtd were highly statistically significant but did not fully explain why the sequence of drought and wetting fosters slev transmission .
however , a probable mechanism is suggested by mosquito collection data taken in the area .
nigripalpus display a bimodal distribution with respect to mean modeled wtd for 3 years ( 1987 , 1989 , and 1990 ) .
nigripalpus males and the individual female age - grades ( data not shown ) . for 1989 and 1990 , the driest years , one of the maxima of cx .
nigripalpus developed sharply at wtds < 1.4 m. none of the other years , including 1987 , had this level of drought or this sharp bimodality .
two inferences may be drawn from these data : either the mosquito population increased at both the driest and wettest times of the year , or during the drought of 1989 and 1990 , mosquitoes congregated in the hammock collection site .
the latter inference is consistent with field observations that the hammocks in indian river county and throughout south florida provide refuge for mosquitoes during periods of drought ( 23 ) .
effect masks the true population dynamics ; however , it illustrates an effect previously reported ( 26 ) , namely , that drought concentrates large numbers of mosquitoes in selected refuges that also harbor large numbers of avian amplification hosts .
total collected female culex nigripalpus plotted as a function of modeled water table depth ( wtd ) ( same day ) .
our findings suggest the following sequence of events for slev transmission in indian river county .
nigripalpus activity to densely vegetated hammock habitats where nestling , juvenile , and adult wild birds are found .
this forced convergence of mosquito vectors and avian amplification hosts provides an ideal environment for the rapid epizootic amplification of slev . when the drought ends and water resources increase , infected mosquitoes and birds disperse from the hammocks , initiating the early transmission phase of the florida slev cycle .
the relationship reported here between modeled wtd and slev incidence has only two explanatory variables and provides a simple predictive framework for forecasting slev transmission . to be sure ,
pre - drought conditions may moderate hammock amplification by increasing or decreasing the overall abundance of mosquito vectors and avian amplification hosts .
data from other florida counties and previous epidemics will have to be examined to elucidate how such population variability affects slev amplification and transmission .
future validation of the model should also include census of wild bird populations and sampling of seropositivity rates in the wild birds .
such data were not available for this study . whether a critical period of drought is necessary for maximum epizootic amplification
the 1986 data , for which the drop in wtd was short - lived and slev transmission was limited , suggest that the longer droughts of 1989 and 1990 were necessary for adequate amplification to produce the mosquito infection rates needed for epidemic transmission . however ,
if a drought persists for too long , the vectors may die , thus precluding slev transmission .
certainly , the biological cycles of virus , vector , and amplification hosts must be coordinated to produce an slev epidemic .
the mechanism of drought - induced amplification described here for indian river county may also operate in regions outside south - central florida that have similar epidemic slev transmission .
in fact , the development of slev epidemics after drought has long been noted in many regions of the united states ( 6,7 ) .
future research will attempt to quantify this relationship between drought , vector , and slev transmission for such regions .
differences in vector species composition , resting habitat availability , and zoonotic host prevalence will no doubt affect transmission rates and the findings of such studies . comparable drought - induced amplification may also occur in other arboviruses .
the recent sporadic outbreak of eastern equine encephalitis virus and west nile virus in northern florida , which came on the heels of a drought broken by the landfall of hurricane allison , suggests that these other disease systems warrant similar study . modeling the hydrologic cycle permits quantification of the relationship between drought and slev transmission and enables real - time monitoring and forecasting of slev transmission incidence . using the hydrology model in conjunction with climate forecast projections
 we used a dynamic hydrology model to simulate water table depth ( wtd ) and quantify the relationship between saint louis encephalitis 
 virus ( slev ) transmission and hydrologic conditions in indian river county , florida , from 1986 through 1991 , a period with an slev epidemic . 
 virus transmission followed periods of modeled drought ( specifically low wtds 12 to 17 weeks before virus transmission , followed by a rising of the water table 1 to 2 weeks before virus transmission ) . 
 further evidence from collections of culex nigripalpus ( the major mosquito vector of slev in florida ) suggests that during extended spring droughts vector mosquitoes and nestling , juvenile , and adult wild birds congregate in selected refuges , facilitating epizootic amplification of slev . when the drought ends and habitat availability increases , the slev - infected cx . 
 nigripalpus and wild birds disperse , initiating an slev transmission cycle . 
 these findings demonstrate a mechanism by which drought facilitates the amplification of slev and its subsequent transmission to humans . 
Modeling Overview and Methods
Data Collection and Analysis
SLEV Transmission Data
Mosquito Data
Model Input and Validation Data
Statistical Analysis
Results
Discussion