IDalIA RAIN FOREST NEAR HILO

Transcription

1 101 GROUNDWATER CONDITIONS IN THE IDalIA RAIN FOREST NEAR HILO Robert D. Doty Institute of Pacific Islands Forestry Pacific Southwest Forest and Range Experiment Station Forest Service, U. S. Department of Agriculture Berkeley, California, stationed at Honolulu, Hawaii Recently, much effort has gone into trying to identify the cause of the 'ohila decline. In the process, many investigators have raised questions about the possible relationship of 'ohi'a decline and the wet soil conditions that are frequently found where decline is severe. Because of scant information on the water conditions in the 'ohila forest, we began a study in 1978 of some aspects of the hydrology of the rain forest on the slopes of Mauna Kea and the Waiakea Forest On the island of Hawaili (lo';rr \ 6.. ':7... ') This paper covers some data collected on groundwater and,offers some new 'insights into the soil-water conditions in the rain forest. Measurements are continuing, and eventually we should be able to quantify the degree of soil flooding, which in turn should help us determine the role it might play in controlling the vegetation. PROCEDURES In the study area, we are not dealing with a deep groundwater aquifer consisting of sands and gravels where water from above percolates slowly, over months or years. Instead, we are concerned with "groundwater," which lies primarily within a shallow, 20- to 90-cm organic muck soil overlying 'ala or pahoehoe lava flows. Annual rainfall in excess of 350 cm maintains the we-tland--cr bog type conditicn-s.gon sequently.,- a pe.rched water.", table lies at or near the soil surface with rapidly changing water levels. At the same time there is also deep percolation to the freshwater lense underlying the Island. In fact, the fate of as much as 50% to 80% of the precipitation must be accounted for as deep percolation. The wells used in this study consist of 2.5 crn diameter plastic tubing which has holes drilled 4 em apart along 45 cm of the lower end. A plastic window screen over the holes and a cap on the end prevent clogging of the well. A cutting tube the size of the wells was used to cut a hole through the soil down to bedrock. Once the wells were placed in the holes, a wooden dowel was placed in each well and a can.placed on the top. Cans are

2 102 used to prev,en:t rainwater f,rom entering tn:e tops of the wells. The wet portion of the dowels in the wells indicates water depth at the time of the ~eekly r~adings. In addition, finely-ground cork is used in the wells to determine the highest water level dur ing the week. The cork floats on the wa ter; if the water rises and then recedes during the week, it leaves a ring of cork around the dowel marking the highest water level during the week. A total of 3S wells are located at seven plots (Table 1). Three plots are in 'ohi I aaecline areas o'n pahoehoe. Two are in healthy areas on p~hoehoe. One plot is in a healthy area on 'ala, and another is an unhealthy area on la'a. Th-e individual well locations represent a wide range of vegetation and soil conditions from very poorly-drained soils to highly-drained soils. Tn all plots where we hav'eestablished wells, we chose the dee,pest soils avail,able in the area to measure groundwater 'c,ond i tions"so the wells tend to represent the deeper soils available. 'l'w,o of the wells have well loggers which provide a continuous recording of water levels. Recording rain gauges are located in or near all plots. Temperature, wind, and humidity are also recorded on one of the plots. In addition, shoot growth ~n several 15hi'a trees on three of the plots is being measured in order to study its possible correlation to wet and dry periods. RESULTS The numbers in Table I are graphically illustrated by the typical plot in Figure 2. The solid horizontal line or zero line represents the ground surface or soil surface and serves as the reference point from which all water levels are measured. Depths to the water surface below ground are given positive values. Consequently, if the water level rises above the soil surface or when a flooded condition exists, the values are negative. All measurements for each well have been used to arrive at the Table I values. The median indicates the level where 50% of the time the water level is higher and 50% of the time the water is lower. The 90% level indicates that the water is at that level or lower 90% of the time, conversely~ it is equal to or higher l,o,% o the..time.the 10% l~yej ind~,9at~~ th~t,the water. is at that level or lower 101 of the time, conversely, -it iseqtial to or higher 90% of the time. Also shown is the highest level recorded and the lowest level recorded by the weekly readings. Plot 1 represents a poorly-drained bog site with a severe '5hi'a decline (Table I). The highest (90%) and the median levels for all wells indicate the wet conditions that exist at least 50% of the time. Wells 3, 4, 5, 7, and 8 are located directly in the bog and have water levels within 7 to 12 cm of the surface 90% of the time. This plot, however, has experienced one dry period when levels dropped to 20 or more centimeters. This occurred during January-February 1980, while Hilo was having a drought. The last column indicates the depth of the wells or

3 103 what the maximum possible drop in water levels could be. Wells I and 6 are on the outer fringes of the bog, and readings there indicate a greater fluctuation in groundwater levels. Plot 2 is also a bog area similar to Plot 1, but two very different wells exist there. well 5 for some reason drains periodically but more than 50% of the time it is completely flooded as indicated by the negative numbers. Well 6 is generally well-drained but physically does not appear to be a welldrained site. Plot 3 is a well-drained healthy ar~a on p~hoehoe. The area is underlain by several lava tubes which have collapsed, and which apparently accounts for the drainage. The site is peiiodically severely flooded for very short periods during major rain storms. Apparently, water is channeled into the area by the lava tubes ~ut, likewise~ the tubes channel the water away rapidly. Well 7 is not really a well but a tube down through a lava tube which carries little water, even during these severe flooding events. Plots 4 and 5 are a pair of 'ohi'a decline (plot 4) and healthy (Plot 5) sites. Within each area are both well-drained and poorly-drained microsites. Wells 3 and 4 on Plots 4 and 5 are within a pocket or remnant of ash or alluvium, which is a well-drained sandy soil not like the organic muck soils which generally are found in the area..the water levels in all of the wells reflected the dry period this winter. Plots 6 and 7 are also a pair of healthy (plot 6) and unhealthy (Plot 7) sites. Because it was difficult to find any soil in the area, the well tubes were placed down through the unconsolidated 'ala as far as they would go without disturbing the site. They rarely held water (Table 1), even during the wettest periods. At least 36 cm of unsaturated profile was above the water level whenever measurements were taken. To illustrate how these fluctuations are distributed in time, Figure 3 shows a plot of weekly readings from wells on Plot 1. The water level appears to fluctuate randomly up and down throughout the year. Occasional dry periods show up as low readings in the wells. The conditions this past winter are - -d-r-am-a-t-ie-a-l-l-y --shown ev-en- tnough this welli s in th-em-iddle 0- a bog. Also, there appears to be a trend or cyclic pattern which transcends any annual cycle. precipitation data have a 12- to IS-year cycle pattern (Doty, in prep.). A closer look at the short-term fluctuations is provided by weekly charts from the recorder on well 4, plot 1 (Figs. 4 & 5). During rain storms, the water in the well rises rapidly and above a certain level surface flow takes over and influences the shape of the curve. It is no longer a groundwater measurement at that point but a measure of surface flow across the area. During dry periods, the well water recedes at a fairly constant rate (Fig. 5). By relating this drop to the relative humidity conditions and deep seepage rates, an estimate of evapotranspiration is obtained.

4 104 CONCLUSIONS We have just begun to compare water levels and other factors. Daily precipitation and daily well readings from well 4 on Plot 1 are closely but not strongly correlated because the changes in well water levels and precipitation are occurring at a shorter time interval than one day. The relationship does indicate that about half of the precipitation is accounted for in a corresponding rise in water levels on the site. Well records cover too short a time define a cycle trend, but the plotted data a trend beyond the normal annual cycle. records from near the coast reported by the defined cycle similar to the 12- to l5-y~ar (USGS, a series of annual reports). period to adequately does appear to reflect The long-term well USGS indicate a wellprecipitation cycle The organic muck soils sampled are saturated to within 5 to 10 cmof the soil surface at least 50% of the time. Some are saturated as much as 90% of the time this close to the surface or even closer. Consequently, regardless of the depth of soil availabla i generally, only a shallow depth of soil is above the water table which could severely restrict root growth in the remaining soil profile. Brief drier per~ods occur any time of the year but predominantly in winter. This past winter, the drought period in Hilo is reflected by the low well reading in all wells. LITERATURE CITED Doty, R. D. tong-term precipitation and groundwater measurements relative to ohia decline in the Hilo area, island of Hawaii. (Manuscript, in preparation). U. S. Geological Survey (USGS). Annual water-data reports, for Hawaii and other Pacific areas.

5 105 TABLE 1. Groundwater levels above or below the soil surface measured in 35 wells on 7 plots, near Hilo, Hawai'i. j.'. Equal to or Equal to or lower than lower than Lowest Well Highest 90% of time Median 10% of time Lowest Possible centimeter Plot ,,, ')" It -.LV -~.LV..JV 77 A.. " '" Plot

6 106 TABLE 1--Continued. Equal to or Equal to or lower than lower than Lowest Well Highest 90% of time Median 10% of time Lowest possible Centimeter plot B B Plot Plot B

7 107 TABLE l--continued. Equal to or Equal to or lower than lower than Lowest Well Highest 90% of time Median 10% of time Lowest possible Centimeter Plot 6 I Plot

8 108 o, 5! 10, 15! Miles FIGURE 1. The 35 wells being used in the study of groundwater conditions are located in 7 plots southwest of Hilo, island of Hawai'i.

9 ~Highest level -10 en (I) -(I) E -----~---- I Soil surface -- 90% of the time oft: fa Median -E: (I) 20 u % of the time 40 ~ Lowest level 50 FIGURE 2. A typical plot of the depth to the groundwater table relative to the soil surface. I-' o ~

10 -10 """" I-' o o Soil Surface " ~ E (,) -U) Q) ~ 10 c: CD "co ~ 0.s:::. Q. CD Q b I I I I I I I I I I,J " 28 Mar 78 6 Jun 78 16, Aug Oct 78 2 Jan Mar 79" 14 May Jul Oct Dec Feb May 8C FIGURE 3. Weekly groundwater measurements from wells, plot 1, from March 28, 1978, through!may 14, 1980.

11 III r----lr-f\--j ~-.J :::.. 25i---t-i---\:---I-+--* l--J > 2.7L-~~~---:=-=--L---=--:-:---L---L _ 10/ / i FIGURE 4. A smoothed.~ecorder trace of the water level in well 4, plot l~ during a rainy period. - ~ 2. > a 2G :: ~ ~..- - ~ ~ ~~ 6/4179 6/5/79 6/6/19 617/ FIGURE 5. A smoothed recorder trace of the,water level in well ~, ~~lotl; during a c per i6d of 4 days with no ra,tn.