No abstract

Mountain block systems are critical to water resources and have been heavily studied and modeled in recent decades. However, due to lack of field data, there is little consistency in how models represent the mountain block subsurface. While there is a large body of research on subsurface heterogeneity, few studies have evaluated the effect that common conceptual choices modelers make in mountainous systems have on simulated hydrology. Here we simulate the hydrology of a semi-idealized headwater catchment using six common conceptual models of the mountain block subsurface. These scenarios include multiple representations of hydraulic conductivity decaying with depth, changes in soil depth with topography, and anisotropy. We evaluate flow paths, discharge, and water tables to quantify the impact of subsurface conceptualization on hydrologic behavior in three dimensions. Our results show that adding higher conductivity layers in the shallow subsurface concentrates flow paths near the surface and increases average saturated flow path velocities. Increasing heterogeneity by adding additional layers or introducing anisotropy increases the variance in the relationship between the age and length of saturated flow paths. Discharge behavior is most sensitive to heterogeneity in the shallow subsurface layers. Water tables are less sensitive to layering than they are to the overall conductivity in the domain. Anisotropy restricts flow path depths and controls discharge from storage but has little effect on governing runoff. Differences in the response of discharge, water table depth, and residence time distribution to subsurface representation highlight the need to consider model applications when determining the level of complexity that is needed. Plain Language Summary Computer models are a commonly used tool to understand and predict the behavior of mountainous watersheds. However, modeling groundwater is uniquely challenging in these systems because groundwater flow is strongly influenced by the geometry of the bedrock, yet observations of depth to bedrock and measurements of hydraulic properties with depth are infrequent. Modelers commonly rely on simplifying assumptions to represent the geologic layering in mountainous watersheds. However, the impact of these assumptions on simulated hydrology is rarely evaluated. Our study compares six commonly used representations of the subsurface to explore how basic choices in model construction influence the resulting hydrology. The six models vary in complexity from no layering at all to more complex variations in soil depth. We compare the behavior of our six cases based on groundwater levels, streamflow, and groundwater flow pathways. We show that streamflow changes the most due to changes in the model layers closest to the surface. The speed of groundwater flow pathways is more sensitive to the properties of layers at all depths. Differences in how streamflow and groundwater flow pathways respond to various representations highlight the fact that good model performance with r...

A total of 1025 coconut palms of the East African Tall variety (EAT) in 41 small-holding plots were examined twice in a survey of Pseudotheruptus wayi damage conducted from July 1992 to January 1993 on the island of Unguja, Zanzibar. The red weaver ant Oecophylla longinodu, the most important predator of P. wayi, was present on 34% and 37% of the total palms in Survey I and 11, respectively. A change in the presence of 0. longinoda on the palms was observed during both surveys. Approximately 50% of the palms were free of 0. longinoda during the surveys. Strong colonies occupied 12% of the palms during both surveys. The remaining palms were either weakly occupied or the predator was found only once. The average annual yield of palms with strong and stable 0. longinoda colonies was 57 nuts while palms without 0. longinoda had an average yield of 41 nuts. Palms with varying degrees of 0. longinoda colonization showed an increasing annual yield with a corresponding increase of 0. longinoda activity. Intense 0. longinoda colonization resulted in a significant reduction of female flower formation as well as less damage due to P. wayi. The number of 7-8-month-old nuts increased significantly with each increase in 0. longinodu activity on palms. The average number of nuts per inflorescences ranged from about 2.7 nuts on palms without 0. longinoda to 4.1 nuts on those with stable colonies. The formation of female flowers per inflorescence, the level of P. wayi damage and the retention of nuts varied seasonally, among the plots, individual palms and also within inflorescences on the same palm. Palms with strong weaver ant colonies displayed less fluctuations in flower production and netset which resulted in a more stable harvest.

No abstract

The weaver ant Oecopliylla l o n g i t d u Latr. attacks the coconut pest Pseudotheraptus wayi Brown. The big headed ant Plieidole n?egucephulu Mayr is the most important enemy of 0. longinoda. After the stop of weed control in a coconut site, the number of palms colonized by 0. longinodu increased from 696 trees in April 199 1 to 1776 palms in April 1992 because P. tnegacephula stopped attacking 0. longinoda and started to forage on the weeds and bushes on the ground. From September 1991 until October 1992 60068 nuts were harvested. The control of P. nzegacephala with Amdro in a neighbouring plot of the same size was less effective because the initial colonization by 0. longinodu in the plot was much lower. These palms produced 28 413 nuts less. It is concluded that reduced weed control can enhance 0. longinodu. 29, 15-33.93-1 13.258-264.