Rivers running dry … Water Resources Authority warns of impact of below-normal rainfall
The Water Resources Authority (WRA) is warning that the below-normal rainfall observed since 2018 continues to affect the water resources of the island. This has led to reduced surface water flows in rivers and streams and less recharge of aquifers in most parts of the country.
And things are likely to get worse, with the next rainy season more than a month away in October.
“The most pronounced reduction in flows was observed in the eastern sections of the island, particularly in the parishes of Portland and St Mary. The rivers most impacted in these parishes are Rio Neuvo in St Mary, Swift and Buff Bay rivers (these rivers depend on rainfall to sustain their flow) in Portland,” the WRA explained in its Stream Flow Bulletin covering the period January-June 2019.
It further noted:
“These rivers are located in the Blue Mountain North Hydrologic Basin (BMNHB). The hydro-geology of BMNHB is Basal aquiclude; therefore, the rivers are mainly rainfall dependent. Flows in January are much below normal for the Buff Bay River, the Swift River, and the Rio Grande, with the Rio Nuevo showing the lowest flow recorded since the installation of the station.
“It is important to note that the major rivers in the BMNHB are perennial as a result of the high levels of rainfall in the Basin. However, small streams will go dry when there are extended periods of reduced or no rainfall.”
For Western Jamaica, the situation is not looking so good either, as the WRA notes in the bulletin.
“There was also noticeable decline in the flows in Great River (St James) and its tributaries (Sevens River). The situation for Great River (Hanover) improved in the latter months, but of note is that the flows in the Great and Sevens River were in some cases the lowest ever recorded by the WRA,” it said.
EXPERIENCING A RAINFALL DEFICIT
The stream flow analyses for February-April for rivers located in the BMNHB were mixed, with the flows for February being normal, which was a response to increased precipitation across the basin. However, for March and April, most of the rivers returned to below normal and much below normal flows. The Hope River, which is in the Kingston Hydrologic Basin (KHB), was much below normal in February and March, responding to the continued below-normal rainfall that is affecting the island.
“What it means is that water-supply systems that are served by surface-water sources such as springs and rivers will be impacted. The systems served by wells, and so on, will be a little bit more resilient and won’t show as much impact,” Managing Director of the WRA Peter Clarke explained. “Essentially, an improvement in that situation can only be expected with rainfall.”
However, the desired level of rainfall to offset a worsening of the drought situation could be some way off, according to director of the Meteorological Service Division Evan Thompson.
He told The Sunday Gleaner that Jamaica is still experiencing a rainfall deficit with little expected before the traditional rainy season. Some areas did experience rainfall recently but not enough to significantly offset the impact of the drought.
Weather systems such as troughs could affect the island, bringing rain and some relief, but the overall outlook is bleak, according to Thompson.
Scrutiny of stre am f low shows cause for concern
According to the Water Resources Authority (WRA), a further scrutiny of the stream flow analysis for January of the Dry Harbour Mountains Hydrologic Basin, the Rio Minho Hydrologic Basin, and the Rio Cobre Hydrologic Basin, where the dominant hydro-geologic units are classified as limestone aquifers, shows cause for concern.
The rivers are mainly groundwater supported, which enables them to maintain perennial base-flow, the WRA noted in its Stream Flow Bulletin covering the period January-June 2019.
Flows for these rivers are in the 25-75 percentile and are within the normal range. However, aquifers are also affected by precipitation levels. This is evident at Martha Brae River at Friendship and Roaring River at Deeside, both of which are groundwater fed but are indicating much-below-normal and below-normal flows, respectively.
The decline in flows in the Blue Mountain North Hydrologic Basin continued to be observed in the months of May and June, with the measured flows continuing to be below normal for all the rivers, with Buff Bay River at Tranquility displaying a new low for the second time this year.
The Hope River in the Kingston Hydrologic Basin (KHB) was normal as the watershed received some rainfall. Flows into the Rio Cobre were much below normal in May but recovered slightly in June as flows were impacted by rainfall in the Basin. The Rio Minho recorded flows below and much below normal for May and June.
RIVERS THAT INDICATE NEW LOW
The flow data collected over the period January to June have shown that water resources have been negatively impacted islandwide. However, the impact is greater on the eastern side of the country, for example, in the Kingston Hydrologic Basin and the Blue Mountain North Hydrologic Basin, where flows in rivers and streams are largely rainfall dependent.
Some of the smaller streams in these basins have become dry as result of severe lack of precipitation, for example, Somerset Falls. In the basins where the rivers are groundwater fed, such as the Rio Cobre and the Dry Harbour Mountains Hydrologic Basins, the observed impact is not as significant as those in the eastern side of the island.
There are rivers that indicate new lows; however, the flows are still sustainable, for example, the Great River near Lethe. While a new low was recorded for this river in January, this did not have a significant negative impact on water supply as the National Water Commission, which uses it as a primary source for Montego Bay and its environs, did not report an adverse challenge to potable water production during the period.