Winter precipitation in two headwaters catchments (elevation ~1600 m) in the rain shadow of the Cascades volcanic arc in south-central Oregon normally falls as snow. However, in water year 2015, winter precipitation fell mainly as rain. An eight year study of the unconfined pumice aquifer allowed inter-annual comparison of groundwater recharge during the freshet and discharge during the growing season. During these water years precipitation ranged from 67% (WY2014) to 132% (WY2017) of the 30 year average, and included the rain dominated winter of WY2015 when precipitation during the water year was 98% of the 30 year average. Change in storage in the pumice aquifer was estimated from thickness of the pumice deposit and depth to water table from the ground surface. Measurements were made where 1) the pumice aquifer was exposed at the surface; 2) where the aquifer was partially eroded and overlain by either alluvium or lacustrine glassy silt to fine sand; 3) fens where the partially eroded aquifer was overlain by peat; and 4) monitoring wells drilled through the pumice aquifer into bedrock. In all settings, groundwater storage in the pumice aquifer following the rain-dominated winter of WY2015 was similar or less than storage following the drought of WY2014 when winter precipitation fell as snow. Storage at the end of WY2014 and WY2015 was the least observed in the eight year study. Winter-time rain during WY2015 produced runoff rather than storage in snow pack. Runoff conveyed from the catchments by flow in stream reaches normally dry from late summer through the winter months. Rain-dominated winter precipitation stresses the perched pumice aquifer. Winter storms starting as rain and turning late to snow and ground-freezing temperatures lead to runoff during the next rain-dominated precipitation event. These patterns produced stream flow in channels that are commonly dry during the winter, reduced near-surface groundwater storage in the pumice aquifer, muted springtime freshet, and stressing of groundwater-dependent ecosystems, forage in meadows, and forest health.
The accumulation of snow during the winter, release of melt water during the spring freshet, and low, scattered precipitation during the summer, characterizes the hydrologic regime in watersheds throughout western North America. Inter-annual variations in these patterns are mirrored in the predicted yield of water from higher elevation catchments to meet demands for wildlife, agriculture, recreation, and city water supplies [
The unconfined pumice aquifer in the Walker Rim study area is an important source of near-surface groundwater in south-central Oregon in the rain shadow of the Cascades Range (
of snow pack during the fall and winter while in WY2015 winter precipitation fell primarily as rain. The primary questions addressed in this paper are: What was the response of the pumice aquifer to rain-dominated winter precipitation? What does this response suggest about groundwater availability in this area under changing climate conditions?
The two forested catchments considered in this study lie at elevations greater than 1590 m within the Walker Rim study area (
The second catchment, the Round Meadow/Sellers Marsh catchment (
most of the catchment lies between 1654 and 1720 m. This closed basin lies between the Deschutes River basin to the north and the Klamath River basin to the south. A ditch connects Sellers Marsh, the lowest elevation wetland in the catchment, and the upper Deschutes River basin; however, evidence of flow in this ditch was not identified during the period of study. The area of the Round Meadow/Sellers Marsh catchment is 64 km2.
The unifying characteristic of the two headwater catchments is the 2.7 m to 3.0 m thick blanket of Plinian pumice fall. The pumice blanket was deposited during the cataclysmic eruption of Mount Mazama in the Cascades volcanic arc at 7700 yr. B.P. [
The unconfined pumice aquifer presents an opportunity to examine groundwater response to inter-annual variations in the form and amount of winter precipitation. The following characteristics of the aquifer support this evaluation. 1) The pumice blanket was laterally uniform with regard to grain size, stratigraphy, and thickness at the scale of the catchments. 2) The aquifer was hosted in deposits that were exposed at the surface and in most areas were capped by excessively drained soil formed from pumice parent material. 3) Groundwater flow paths within the pumice blanket were controlled by low-relief pre-eruption topography. Disruptions of the pumice aquifer occurred where post-eruption shallow lakes and streams reworked the upper part of the pumice deposit, where iron oxide precipitation has reduced permeability within the aquifer, and where streams have eroded through the pumice deposit to the pre-eruption surface [
Groundwater data were gathered from 22 nested piezometers installed by The Nature Conservancy and Fremont-Winema National Forest at three fens [
From WY2011 through WY2015 and WY2017, depth to water table was measured between 1-June and 4-June (herein referred to as 1-June) and at the end of the water year (late September) or early in the next water year (October). In addition, depth to water table was measured during the growing season at irregular intervals. The depth to water table was compared to the thickness of the Plinian pumice fall and, where present, overlying water-lain deposits. Thicknesses of the pumice deposit were determined from auger holes that penetrated the lower contact or were estimated on the basis of the internal stratigraphy of the pumice deposit where the auger hole stopped short of the lower contact. From these data the saturated thickness of the pumice aquifer and changes in storage were estimated.
Air temperature and dew point were measured at least every 60 minutes at 1.8 m above ground surface, and 50 cm and 95 cm below ground surface in the upper Jack Creek catchment at an elevation of 1642 m. Records are available from 10-July-2010. Sensors used for these measurements were Hobo U23 Pro v2 temperature/relative humidity data loggers. The instrumented site was in a stand of Lodgepole pine trees in mottled shade. Precipitation data were available for the Chemult Alternate SNOTEL Site since WY1981 (http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=395). The elevation of this site is 1478 m, approximately 100 to 300 m lower than elevations in the study area. The SNOTEL Site is located within 12 to 18 km of most sites in the upper Jack Creek and Round Meadow/Sellers Marsh catchments.
Groundwater in the pumice aquifer resides in two distinctly different settings: 1) in pore spaces between pumice grains and 2) in vesicles within pumice grains. Groundwater within the pore spaces between pumice grains moves freely. Weatherford and Cummings [
The annual recession of the water table between 1-June and the end of the water year was approximated as linear based on field measurements from ground surface to water table. The slope of the line was determined for each water year and where field measurements for 1-June or September 30 data were not available. Depth was estimated from the linear relation determined from available field data. 1-June was selected for scheduling convenience for field work, field conditions that allowed access throughout the piezometer network, early timing in the growing season at this elevation, and precipitation patterns associated with the summer (isolated showers with little impact on recharge of the aquifer). Earlier access was possible inWY2014 (19-20-April) and WY2015 (7-March and 18-19-April) because of low snow accumulation during the winter. In some settings, these pre-1-June measurements did not fall on the linear trend that characterized recession of the water table between 1-June and the end of the water year. The differences were interpreted to reflect incomplete adjustments of the groundwater system relative to the freshet-related time of recharge. Likewise, late October measurements deviated from the linear trend in WY2015 when the onset of widespread fall precipitation occurred before final field measurements were performed.
Relatively few auger holes drilled where the Plinian pumice fall was exposed at the surface encountered a water table. At the eleven sites where groundwater was detected, open-ended PVC pipes (244 cm lengths) with slot cuts were inserted in the auger holes. At these eleven sites at least some of the upper pumice unit had been eroded.
The Forest Service local road 510 (FSLR 510) piezometer (A on
The hydrograph for the FSLR 510 site is presented in
Water year | Total precipitation (cm) | April 1 SWE (cm) | April 1 total precipitation (cm) | % of water year precipitation by April 1 |
---|---|---|---|---|
2010 | 60.2 | 6.4 | 41.4 | 69 |
2011 | 82.6 | 28.4 | 67.6 | 82 |
2012 | 71.9 | 19.3 | 60.4 | 84 |
2013 | 62.0 | 0.0 | 46.2 | 74 |
2014 | 46.7 | 0.0 | 36.8 | 79 |
2015 | 68.3 | 0.0 | 51.6 | 76 |
2016 | 78.2 | 22.9 | 71.9 | 92 |
2017 | 92.2 | 23.6 | 80.3 | 87 |
30-year average | 69.8 |
winter precipitation, over 20 cm of SWE on 1-April, and greater than 80% of annual precipitation received by 1-April, behave similar to the “wet” years.
The recession rate of the water table in cm/day between 1-June and 30-September was greater in water years when the water table on 1-June was within the main rooted zone. In contrast the recession rate was lower in water years when the water table was below the main rooted zone on 1-June. Higher recession rates (e.g. 0.61 cm/day in WY2011 when water table was 48 cm below ground surface on 1-June) occurred in the “wet” years and WY2016 and WY2017. The recession rates were lower in “dry” years and WY2015 (0.38 cm/day when water table was 105 cm below ground surface on 1-June). Thus, recession rates are sensitive to the change in storage produced by aquifer recharge during the freshet.
Recharge of the pumice aquifer was estimated from data for 1-October to 1-June. The greater recharge occurred in WY2011 (84 cm) and WY2016 (83 cm) with less in WY2012 (63 cm), the “wet” years. Less recharge occurred in WY2013 (41 cm) and WY2014 (43 cm) the “dry” years. Recharge in WY2015 (56 cm) was intermediate between “wet” and “dry” years. Storage was lowest on 30-September in WY2014 and WY2015 when 30 percent and 34 percent, respectively, of the pumice aquifer was saturated. The greatest saturated thicknesses on 1-June were observed in WY2011 and WY2017 when the saturated thickness was 79 percent and 76 percent, respectively.
A second site that illustrates groundwater response in this group was located near FSLR 460 (B on
The second group of piezometers includes those installed in auger holes used to determine stratigraphy in ephemeral stream valleys and lake beds cut into the pumice deposit. This group also includes three sites where the uneroded pumice deposit was covered by alluvium. After the eruption of Mount Mazama the drainage network began to evolve. Streams followed the pre-eruption landscape
template and eroded into and locally through the pumice deposit. Five to 10 cm of lag sand comprised of angular to sub-angular crystals (phenocrysts), and sub-rounded lithics overlie the scour surface. Following this early stage of erosion, glassy silt, fine-grained glassy sand, and rounded granules to pebbles of pumice were deposited forming flat bottomed valley floors cut into the pumice deposit. In these settings the aquifer was the remnant of the lower pumice unit and the lag sand. A second feature of the drainage network consisted of shallow lakes that ponded upstream from bedrock-lined constrictions in the pre-eruption landscape. The Round Meadow system illustrates this environment [
Forty-four single piezometers were used to monitor ground water levels and two sites have been selected to illustrate storage in these settings. The first was an alluvial site east-northeast of the Johnson fen and the second was a lacustrine site in Round Meadow (C and D, respectively,
Settings where alluvium overlies the partially eroded pumice aquifer are illustrated by a low-relief valley that extends southward from the Wilshire fen at its north end, passes by the Johnson Meadow fen, to its confluence with Jack Creek (
Two distinct tributary valleys enter the north-south valley, one from the east and one from the west (
the maximum depth of the piezometer (167 cm).
The depth to water table on 1-June follows patterns similar to those observed at sites described in Section 4.1. In “wet” years (WY2011 and WY2012) the water table on 1-June was above or near the surface while in “dry” years (WY2013 and WY2014) the water table on 1-June was greater than 80 cm below ground surface. Again, WY2015 follows the “dry” pattern, but has an even deeper water table on 1-June than the drought year of WY2014. The pattern for WY2016 and WY2017 were similar to the “wet” years, but the water table did not reach the surface. The frequency of measurements in WY2011 allowed a change in rate of recession to be detected that mirrored the depth of water table relative to the depth of the contact between fine-grained alluvium, lag sand, and pumice deposit. From 1-June to 12-August the rate of decline was 1.3 cm/day while the water table was in the heavily rooted zone hosted by alluvium. From 12-August to 20-October the rate of recession was 0.7 cm/day while the water table was hosted within the pumice aquifer. Recharge in “dry” years was not enough to raise the water table into the alluvium, rather the water table remained within the pumice deposit during those years. The recharge was even less in WY2015. The 1-June position of the water table within the pumice aquifer in these three water years (−81 cm in 2013; −80 cm in 2014; −87 cm in 2015) was consistent with lower recession rates during those years (0.67 cm/day in 2013; 0.57 cm/day in 2014, 0.50 cm/day in 2015).
The second site that illustrates this stratigraphic relation is located in Round Meadow (D in
The piezometer selected from this site was installed in the southeastern corner of the meadow where alluvial fan deposits (40 cm thick) overlie lacustrine sediments (48 cm thick) which, in turn, overlie the pumice aquifer (~190 cm thick;
Although the hydrograph in
cm/day in WY2013. This is distinct from most alluvial/lacustrine sites where the recession rate in WY2011, a “wet” year, is greater than those in “dry” years such as WY2013. 3) In WY2015, the early season recession rate was considerably lower than near the end of the water year. Access to the site was possible on 7-March-2015 in the morning while roads were still frozen. Between this date and 3-June, the recession rate was 0.34 cm/day with the water table within 3 cm of ground surface on 7-March when the ephemeral stream was still flowing. The recession rate was greater from 3-June to the end of the water year with an estimated recession rate of 1.19 cm/day. 4) The depth to water table at this site (
A relation between recession rate of the water table and subsurface storage on 1-June is suggested by data from all piezometers measured in this study. These findings are consistent with findings reported by Garcia and Tague [
surface water was present at sites plotted in
These relations reflected the difference in function of the pumice deposit and reworked deposit. The function of the pumice deposit was primarily storage of groundwater. The function of reworked deposits was conveyance of surface water from the system.
The internal stratigraphy of five fens was described by Cummings et al. [
An erosion surface marked the contact between the peat or organic-rich sandy silt confining layer and the underlying pumice aquifer. This contact cut downward through the pumice deposit and locally to the pre-eruption surface. In some fens, piping features rose from the pumice aquifer through the peat confining layer and discharged groundwater to low-volume streams which flowed across the peat surface. Focused discharge from these streams and diffuse discharge through the fen surface accumulated in small pools or streams at the toe of the fen. This discharge contributed to perennial stream flow in the upper reaches of Jack Creek. However, locally, the discharge ponded against low-relief berms that marked the surface boundary between the fen and neighboring alluvium-floored, ephemeral stream valleys [
Nested piezometers were installed in three fens (Wilshire, Johnson Meadow, and Dry Meadow) by The Nature Conservancy in collaboration with the Fremont-Winema National Forest [
the end of the water year with the greatest recession noted in WY2014. The late season recession of the water table in WY2015 was similar to the drought year of WY2014.
Ten wells were drilled by the U.S. Forest Service to monitor ground water levels [
At the three fens, the wells were upslope from the “wetland on dry ground amongst Lodgepole pine trees” [
Augered interval (m) | Cored interval (m) | Piezometer screened interval (m) | Water table* on 10-Oct-2010 (m) | |
---|---|---|---|---|
Dry Meadow 1 | 0 - 3.81 | 2.29 - 3.81 | 1.13 | |
Dry Meadow 2 | 0 - 3.05 | 3.05 - 15.24 | 10.67 - 15.24 | 14.90 |
Dry Meadow 3 | 0 - 3.05 | 3.05 - 27.28 | 22.23 - 27.28 | Dry |
Johnson Meadow 1 | 0 - 6.70 | 1.83 - 3.35 | 1.31 | |
Johnson Meadow 2 | 0 - 7.61 | 7.61 - 13.72 | 10.67 - 13.70 | 13.56 |
Johnson Meadow 3 | 0 - 7.61 | 7.61 - 29.20 | 24.32 - 28.90 | Dry |
Wilshire 1 | 0 - 4.57 | 1.83 - 2.03 | 1.42 | |
Wilshire 2 | 0 - 9.14 | 9.14 - 18.29 | 13.40 - 18.29 | 17.39 |
Wilshire 3 | 0 - 9.14 | 9.14 - 31.85 | 27.25 - 31.70 | Dry |
Section5 | 0 - 6.10 | 6.10 - 15.18 | 1.52 - 6.10 | 1.26 |
*Depth to water table measured from the top of the piezometer [
basalt (Johnson Meadow) or unwelded pyroclastic flow and basaltic hydrovolcanic tuff (Wilshire). In all cases, the productive aquifer was the pumice aquifer underlain by lower permeability regolith or bedrock units.
Depth to water table from the top of the piezometers has been monitored since installation.
In general, for monitoring wells, the greatest recession occurred in WY2014 and WY2015 when water tables on 30-September were 25 to 37 cm lower, respectively, than the first measured date (10-Ocotober-2010) at the three fens. The recession was less in the single isolated well where the water table declines were only 11 and 14 cm, respectively, compared to 10-October-2010.
The two catchments had relatively minor surface water resources. The largest
area of surface water was Sellers Marsh in the Round Meadow/Sellers Marsh closed basin, but this site was not available for study. Round Meadow in the same closed basin had an area of approximately 0.42 km2 when it was flooded. By late summer in “dry” years surface water was confined to the largest canals that were constructed in the 1960s to drain the wetland. Surface water discharge to Sellers Creek occurred when the depth of water in the meadow exceeded the elevation of the bedrock knickpoint at the head of Sellers Creek. The perennial reach of Sellers Creek extended for approximately 6.5 km from the knickpoint and was maintained by discharge from the meadow and from springs discharging from fractured welded tuff that formed the knickpoint [
Perennial flow was also present in approximately 11 km of Jack Creek. Base flow in the perennial reach is maintained by discharge from springs issuing from contacts between basalt flows and between lava flows and lower permeability bedrock units. In this environment, springs are noted for discrete points of discharge, but in many cases the groundwater discharge was into the pumice aquifer and released through fens. Base flow in Jack Creek was measured at FSCR 9418 (
Perennial flow was also present in channel lengths of 10 m to 100 m near fens and low volume springs. Discharge from springs in the Round Meadow system [
The ephemeral drainage network conveyed water during the freshet in “wet”
Discharge on Jack Creek at FSCR 8821 | |
---|---|
Date | Discharge in m3・s−1 |
2-June-20121 | ~4 |
29-June-2012 | 0.52 |
6-July-2012 | 0.13 |
2-August-2012 | Dry |
8-September-2012 | Dry |
20-October-2012 | Dry |
27-April-2013 | 0.87 |
1-June-2013 | 0.25 |
7-July-2013 | Dry |
1-June-2014 | 0.12 |
7-July-2014 | Dry |
26-September-2014 | Dry |
10-December-20152 | Dry |
27-December-20153 | 3.0 - 3.5 |
21-January-20154 | 0.8 - 0.9 |
18-April-2015 | 0.68 |
3-June-2015 | 0.18 |
13-August-2015 | Dry |
27-September-2015 | Dry |
24-June-2016 | 0.29 |
14-October-2016 | Dry |
4-June-2017 | 3.15 |
1Flow meter not functioning properly, discharge estimated from depth of water column, estimated velocity, and comparison to discharge measured under similar conditions. 2Forest Service personnel fide T. Simpson, 31-December-2014. 3Discharge estimated from water depth of 34.7 cm in culvert and velocity estimated from similar conditions on other dates. Depth measured by T. Simpson, 27-December-2014. 4Discharge estimated from water depth of 22.5 cm in culvert and velocity estimated from similar conditions on other dates. Depth measured by T. Simpson, 21-January-2015.
years (
Change in storage in the pumice aquifer follows annual precipitation patterns. During the water years considered in this study, maximum storage was attained in April/May followed by steady decline to minimum storage in September/October. The decline in storage coincided with the six months, April through September, when evapotranspiration rates were highest [
In addition to precipitation data for the Chemult Alternate SNOTEL station during the study years, records of ground temperature were available within the study area at 50 cm below ground surface within the pumice deposit (Section 4.1 setting). An Onset temperature/relatively humidity logger recorded conditions at least every 60 minutes.
Water year | Date when warming started at 50 cm | Temperature in ˚C | 1-June Temperature in ˚C |
---|---|---|---|
WY2011 | 8-May | 0.66 | 3.8 |
WY2012 | 28-April | −0.004 | 4.4 |
WY2013 | 1-April | −0.26 | 6.4 |
WY2014 | 29-March | −0.09 | 6.7 |
WY2015 | 15-March | 0.55 | 8.3 |
WY2016 | 9-April | 0.27 | 6.8 |
WY2017 | 22-April | 0.47 | 7.7 |
Lodgepole pine forest. Short-lived (often less than 12 hours) negative deflections in the curves were produced during rain events when water that was colder than the ground migrated downward along the PVC pipe and chilled the cavity where the sensor was located. These deflections occur during approximately the first 75 days of each water year, but are particularly common in WY2015 (
depth (15-March) and the highest ground temperature on 1-June (
The occurrence of groundwater and the flow pathways it follows in volcanic rocks are controlled by structure, the stratigraphy and physical properties of volcanic units and associated volcaniclastic sedimentary deposits, and the geomorphologic evolution of the volcanic landscape. Examples of the complexities that arise in volcanic systems are described throughout the world (e.g. [
The eruption of Mount Mazama mantled the low relief landscape of the study area with approximately 3 m of pumice. The aquifer that developed in the pumice layer is relatively isotropic, but has been modified 1) by erosion followed by deposition of alluvium in flat-bottomed ephemeral stream valleys, 2) by erosion followed by lacustrine sedimentation in areas where shallow lakes ponded shortly after the eruption, 3) by erosion followed by peat and organic-rich sediment accumulation in fen environments, and 4) by iron oxide precipitation within the aquifer that locally restricted lateral flow [
The expected annual pattern of snow accumulation during winter months, rapid springtime melting and recharge of the pumice aquifer, and discharge by evapotranspiration during the summer growing season did not occur in WY2015 when winter precipitation fell primarily as rain. Recharge of the pumice aquifer in WY2015 (Figures 3-6) when cumulative precipitation was above average (
WY2011 | WY2012 | WY2013 | WY2014 | WY2015 | WY2016 | WY2017 | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
FSLR 510 piezometer | |||||||||||||
10/1 | 6/1 | 10/1 | 6/1 | 10/1 | 6/1 | 10/1 | 6/1 | 10/1 | 6/1 | 10/1 | 6/1 | 10/1 | 6/1 |
43% | 80% | 46% (40%) | 73% | 42% (38%) | 60% (67%) | 37% | 56% | 30% | 54% (62%) | 35% | 81% | 41% (37%) | 76% |
0.58 - 0.61 cm/day | 0.38 - 0.50 cm/day | 0.56 cm/day | ND | ||||||||||
FSLR 460 piezometer | |||||||||||||
44% | 27% | 37% (43%) | 24% | 40% (43%) | 24% | 47% (50%) | 31% (30%) | 50% | |||||
0.30 - 0.37 cm/day | ND | ||||||||||||
East tributary valley, easternmost piezometer | |||||||||||||
51% | 100% | 54% (50%) | 97% | 46% | 71% | 41% | 71% (72%) | 44% (39%) | 68% (73%) | 45% | 87% | 50% (46%) | 97% |
1.15 cm/day | 0.50 - 0.67 cm/day | 0.82 cm/day | ND | ||||||||||
Southeast Round Meadow piezometer | |||||||||||||
99% | 69% (62%) | 100% | 64% (59%) | 97% | 45% (57%) | 83% (96%) | 47% | 89% (99%) | 34% | 100% (100%) | 100% | ||
0.88 cm/day | 0.75 - 1.20 cm/day | ND | ND |
Percentages in parentheses are saturated thickness on dates in October after the start of the water year and on dates in April or May. Highlighted percentages (red) for the east tributary valley and southeast Round Meadow piezometers indicate the water table was within the pumice layer beneath either alluvium or lacustrine sediments on the date of measurement. ND = not determined.
In this discussion, we will consider the inter-annual variations in storage in the pumice aquifer in relation to precipitation patterns. The focus will be on three water years; 1) WY2013, 2) WY2014, a year of severe drought, and 3) WY2015 when winter precipitation was rain dominated. This is followed by discussion of rain- versus snow-dominated winter precipitation in this landscape.
Water year 2013 was the first of three water years when SWE was zero and the percentage of annual precipitation received was below 80% on 1-April. This water year was preceded by two water years when annual precipitation was above the 30-year average (
Sixteen (16) percent of the annual precipitation fell during the last six months of WY2012 (
In fen environments, groundwater levels peripheral to the main area of upflow experienced declines in water levels (
WY2014 was the third driest year on record at the Chemult Alternate SNOTEL Site. During that water year the severe drought conditions experienced in California extended northward into the study area. The cumulative precipitation curve for the Chemult Alternate SNOTEL Site persisted at record low values between the start of the water year and about the middle of February (
The study area followed precipitation patterns throughout the western United States where there was exceptionally low snowpack in WY2015 [
Although precipitation, mainly as rain, was greater than the 30-year average during the first 4.5 months of the water year, storage in the pumice aquifer on 1-June was similar to storage on 1-June in WY2014 when record low precipitation was received as snow during the same time span (
The rain-dominated winter of WY2015 in the study area coincided with snow drought in the western United States. Snow records west of 115˚W for 1-April found the lowest ever recorded SWE at 81% of stations with at least 40 years of record [
Although changing climate in the Pacific Northwest is expected to produce shifts toward less snow, more rain, and early snowmelt [
The three water years marked by zero SWE on 1-April at the Chemult Alternate SNOTEL Site, relatively lower recharge of the pumice aquifer, and the lowest storage at the end of the water year are different in terms of snow- versus rain-dominated winter precipitation. In WY2013 and WY2014 the annual precipitation was below the 30-year average, but fell primarily as snow. Low recharge of the aquifer during the freshet was related to low snow pack. In WY2015, winter precipitation was above the 30-year average through much of the winter, but primarily as rain. Stream flow leaving the upper Jack Creek basin in December and January (
Potentially, climate change scenarios of more frequent snow- and rain-dominated and rain-dominated winters [
Groundwater storage in the unconfined and partially confined pumice aquifer follows annual precipitation patterns with the maximum storage following the spring freshet and minimum storage near the end of the water year and start of the next water year.
The highest recession rates of the water table occurred where alluvium overlies the partially eroded pumice aquifer and the water table on 1-June was within the rooted zone. Lower recession rates were noted where the 1-June water table was within the pumice aquifer but below the main rooting zone for most plants. Lower recession rates were noted in “dry” years when the 1-June water table was deeper in the pumice aquifer than in “wet” years when the 1-June water table was near the surface.
Water years characterized by deeper water tables on 1-June received less than 80% of precipitation by 1-April and had zero SWE on 1-April.
Maximum storage and change in storage during the growing season in the pumice aquifer following the rain-dominated precipitation pattern of WY2015 was similar to patterns observed in the “dry” water years of WY2013 and WY2014 even though total precipitation was near the 30-year average.
Rain-dominated winter storms followed by cold temperatures in WY2015 produced freezing of the ground surface and contributed to runoff during the winter months. The lack of water storage in snowpack contributed to weak recharge of the pumice aquifer during the freshet, anomalously low stream flow on 1-June, and deep water tables throughout the growing season. These factors contributed to drying of peat layers and local desiccation in fens, reduced forage in meadows, and reduced discharge from springs. Similar patterns were observed in the severe drought of WY2014.
Cummings, M.L. and Eibert, D.A. (2018) Winter Rain versus Snow in Headwater Catchments: Responses of an Unconfined Pumice Aquifer, South-Central Oregon, USA. Journal of Water Resource and Protection, 10, 461-492. https://doi.org/10.4236/jwarp.2018.104025