ABSTRACT:

In Hawaiʻi, groundwater draining from wetter, high-recharge uplands provides virtually all drinking water and delivers freshwater nutrients (as well as contaminants) to ecosystems in the nearshore environment. Despite the importance of groundwater, its hidden nature makes it difficult to observe and quantify, which results in a poor understanding of its spatiotemporal dynamics from the uplands to the coast. We combine a storage-discharge approach with a long-term (7+ years) water balance to quantify the relationship between groundwater and stream discharge in 11 (USGS-gaged) watersheds across the Hawaiian Islands. We then use each watershed’s unique storage-discharge relationship to resolve daily estimates of catchment storage and the ‘groundwater leakage’ flux emanating from the watersheds. The resolved mean specific daily leakage is consistent with compiled measurements of submarine groundwater discharge (SGD) at the coast downstream of the 11 watersheds and reflected in the hydraulic connectivity between leakage and head response in a basal aquifer. Example groundwater leakage hydrographs from Hālawa (Oʻahu) and Waiākea (Hawaiʻi) watersheds resolve temporal dynamics of flow moving below the urban centers of Honolulu and Hilo, where contaminants enter aquifers, poison drinking water, and degrade coastal environments. By resolving groundwater leakage at a daily timestep, we reveal a previously hidden portion of the hydrologic cycle that can inform modeling of water resources and water quality in leaky watersheds in Hawai‘i and beyond.

ABSTRACT:

The high-Andes biome, which includes grasslands, peat-forming wetlands known as bofedales, and native forest, provides vital ecosystem services, such as water regulation, to millions of people living downstream. Whereas recent studies have highlighted the hydrologic regulation of native grasslands and bofedales, and quantified reductions in water yield resulting from afforestation with exotic species, little is known about changes to water resources resulting from conversion of native grasslands to Polylepis forests. We measured hydrologic fluxes from the root zone to catchment scale in Jarava Ichu grasslands, young Polylepis (afforested over 0.568 km2 in 2016-2017), and a mature Polylepis forest in the seasonally dry puna biome of the Cusco region, Perú. Compared to grasslands, the Polylepis root zone experienced more drying, reached field capacity later in the wet-up, and passed an average of 41 % less water annually to the underlying aquifer. Average annual evapotranspiration in young and mature Polylepis was 1.9 and 3.2 times larger than in J. ichu grasslands, respectively. Despite the varied ecosystem services provided by native forests to the local and regional communities, we predict a 10-11 % reduction in water yield from the Upper Ramuschaka Watershed (2.12 km2) if the afforested Polylepis grows to maturity. The results presented here will guide hydrological modeling in the region and inform community-led discussions assessing the relative importance of the ecosystem services provided by the varied land covers of the puna biome.

ABSTRACT:

Study region:
Humid puna of the Central Andes, Perú

Study focus:
Bofedales, or peat-forming wetlands, are a characteristic feature of the humid puna
- a high elevation, seasonally dry grass- and shrub-land throughout the Central Andes. Despite
the hydrologic importance of the humid puna for downstream communities, and the inference
that bofedales play an important role, few studies have explored the hydrology of this ecosystem,
and none have quantified bofedal water yield to streams. We designed a 3-year study in
the Upper Ramuschaka Watershed (URW), a 2.12 km2 humid puna catchment sustaining a
perennial stream used for irrigation downstream. We monitored hydrologic fluxes through the
URW, periodically measured discharge in 19 nested subbasins across wet and dry seasons, and
characterized the structure, hydraulic properties, and storage capacity of four bofedales.

New hydrological insights for the region:
Unit runoff is consistently higher in subbasins with
greater bofedal coverage. High porosity peat fills in the wet season via groundwater recharge
and drains slowly through underlying layers with low hydraulic conductivity. Bofedales cover
11.6% of the URW and store 105,000 10,000 m3 of water seasonally. In the dry season,
bofedales yield 49 5 mm to streams, equivalent to 20 – 98% of the URW’s dry season runoff.
Bofedales regulate drainage from the humid puna to downstream communities and are therefore
vital to local and regional water security.