Drought in Delaware

Highlights:

• Statewide precipitation is right around normal over the last 30 to 90 days. Significant deficits only show up at the 12 month timeframe or longer due to the record dry fall of 2024
• Soil moisture in the top meter of soil is in the normal range for this time of year
• Stream conditions are below normal for this time year
• Groundwater conditions vary throughout the state, with some locations having near normal levels, while others are below normal.
• July is expected to be slightly warmer than normal with above normal precipitation

Statewide Drought Discussion

The past week has been quite warm for Delaware.  Monday and Tuesday’s (23rd and 24th) high temperatures were well into the 90’s with abundant sunshine and plenty of humidity.  Despite the humid conditions, which limits the evapotranspiration rate, many locations in Delaware still saw daily reference evapotranspiration (RefET) values above 0.25 inches several days last week.  Figure 1 shows the 7-day total RefET for the DEOS Network from June 17th to June 23rd.

Most locations in northern Delaware lost between 1.0 and 1.25 inches of RefET, while most of Sussex County saw around 1.5 inches of RefET.  These are certainly higher than average values for this time of year, but with the summer solstice taking place on June 20th, and solar conditions being at the annual maximum, high RefEt rates are not that unusual.  Hopefully next week will be a little cooler and we can slow down the rate of water being lost to the atmosphere. 

As we round out the month of June and reach the “halfway point” of 2025, it’s a good time to see how our short and long term precipitation trends are going.  Table 1 shows the 1, 2, 3, 6, and 12 month precipitation departures through June 23rd (relative to the 1991-2020 climate normals) for Wilmington and Georgetown.

Overall, both stations are close to normal for the 6-month and shorter time frames.  Only the 12-month time frame shows a significant departure from normal.  This is mainly because of the record dry fall Delaware experienced in 2024.  This also explains why our groundwater levels have taken so long to recover, as those levels tend to be better correlated with longer term precipitation departures, such as the 6 and 12 month time frames. 

Meanwhile, soil moisture, for most of Delaware, is very responsive to short term (1 month or shorter) precipitation trends.  With recent precipitation lagging somewhat, soil moisture levels have regressed back to the normal range for this time of year.  However, even with June precipitation at that level, soil moisture in the top 1 meter of soil is still well within the normal range for this time of year (Figure 2). 

Despite the precipitation we had last week, the streamflows in the northern part of the state are flowing below their respective medians for this time of year.  The daily flows for all of the major streams in northern New Castle County have been flowing below the 25th percentile since June 21st. Significant precipitation will be needed to keep flows from decreasing as the heat wave continues this week. Flows on streams in Kent and Sussex Counties are well below their median values.

Chloride concentrations in the Christina River near Newport remain well below the first threshold of concern (250 ppm) at 14.56 ppm. The last time the chloride concentration was above 250 ppm was during the first week of March (263.2 ppm).  In general, chloride conditions have been very low thus far in June. Chlorides on the Delaware River have been well below the 250 ppm threshold since at least mid-February.

Groundwater conditions in the unconfined aquifer (i.e., surficial aquifer) in northern New Castle County continued to recharge slightly over the past week.  DGS well Db24-18 (located just south of Newark) continued its slow recovery from the drought earlier this year.  The groundwater level in this well remains at 12.4 ft below land surface (Table 2), which is about 0.5 feet above the below normal level for this time of year. The water level in this well typically ranges between 9.4 and 12.9 feet below land surface during June.  Elsewhere, groundwater levels in the unconfined aquifer throughout the state are a mixed bag, with some locations showing near normal groundwater levels (southern New Castle County and central Sussex County) and other locations showing below normal groundwater levels (southern Kent County).  

Weather and Climate Outlook:

 The weather pattern over the next week should remain warm, but probably not as warm as it has been recently.  Just as importantly, we should see improved chances for precipitation over the next week or so as the ridge of high pressure that caused the hot, dry conditions we experienced the last several days slides off to the south and east.  The latest quantitative precipitation forecast (QPF) from the National Weather Service shows the potential for 1-1.5 inches of precipitation over the next 7 days.  As we look at July, NOAA’s Climate Prediction Center (CPC) is calling for slightly above normal temperatures and above normal precipitation (Figures 3 and 4) for Delaware.