During the 2019 growing season, Dr. Eric Hunt of Atmospheric and Environmental Research, Inc. will be providing weekly updates of the soil moisture index (SMI) from the Noah-MP land surface model in the NASA LIS framework for the eastern 3/4 of the U.S. where row-crop agriculture is more common. The Evaporative Stress Index (ESI) is now included in our analysis. The analysis is intended to provide the larger agricultural and meteorological communities insight as to areas where soil moisture is excessive or deficient compared to average for that location and what that may mean for impacts. It is my goal that these maps can be an early warning signal for flash drought development or where flash flooding could be likely in the coming week if heavy precipitation materializes. Please be advised that the SMI should be viewed as complementary, not a substitute, to the U.S. Drought Monitor (USDM) and that declarations of drought for a particular location should never be based on the SMI alone.
This blog post was partially supported by NASA grant NNH16CT05C.
Figure 1. The Soil Moisture Index (SMI) for the 7-day period ending 28 September 2019. Results are based on output from the 0-1 m (surface to 3.23 feet) layers in the Noah-Multiparameterization (Noah-MP) land surface model. Noah-MP is run in the NASA Land Information System (LIS) framework with the North American Land Data Assimilation Version 2 (NLDAS-2) forcing dataset. The SMI calculation is based on the soil moisture index created in Hunt et al. (2009) such that ‘5’(dark green) is the wettest and ‘-5’ (dark red) the driest for the period of record. The period of record used calculate the SMI for the current map is 1979-present.
Figure 2. Same as Figure 1, except Noah-MP is run with a dynamic vegetation option, instead of a climatologically driven leaf area index (LAI).
Figure 3. Comparison of this week’s SMI map the last three week’s SMI maps.
Much of the eastern U.S. is either in flash drought or headed toward one, as evidenced by the amount of abnormal dryness and drought on the current U.S. Drought Monitor. This is also confirmed by the SMI in Figures 1-3, with the dynamic vegetation simulation depicting particularly grim conditions. It is my opinion that the dynamic vegetation simulation is a little too aggressive with the amount of area at -4 or lower. However, much of this region (and the eastern 2/3 of the U.S. in general) had temperatures that were closer to August normal than September. Thus, the evaporative demand also would be much above normal, exacerbating a situation where rainfall has been limited. As always the maps reflect conditions at the end of last week, so not truly in real-time. But given the record heat that has occurred over the region where flash drought is occurring or imminent, what is now reflected in Figure 2 (i.e., the dynamic vegetation simulation) may well be the SMI in Figure 1 next week. Note that drought and low SMI values are found over much of Texas and western Oklahoma, the exception being coastal southeast Texas, which is a reflection of remnants of Tropical Storm Imelda last month. It should be noted that had Imelda actually been able to move northeast (from southeast Texas), a lot of this discussion about flash drought would be moot.
Further evidence of the rapid deterioration in the eastern U.S. is shown by the Evaporative Stress Index (Fig. 4) and by the 1-month change map from the National Drought Mitigation Center in Figure 5. Note that abnormal dryness is not considered drought but if you are currently in this category in the eastern U.S., official classification of drought may not be far off. I didn’t see a soil moisture percentiles forecast from NASA SPoRT yet this week but this nifty animation from SPoRT shows how much root zone soil moisture has declined over the past month in the southeastern U.S.
Figure 5. The 1-month change in drought categorization. Figure courtesy of the National Drought Mitigation Center.
Meanwhile in the north central U.S., rainfall in recent weeks has been abundant. Some of this precipitation in Illinois and eastern Nebraska was welcome, but in many locations it has been excessive. The cold front that has taken its sweet time moving across the northern and central U.S. has brought additional precipitation in places that were already moist or too wet. I expect next week’s SMI map to look somewhat similar to what we had back in May- lots of green in the Western/Central Corn Belt and Central/Northern Great Plains and lots of red in the east.
For crops in the Corn Belt, the warm September was a godsend for late planted crops and I expect much of it to still make it to maturity. The next round of colder air moving into the north central U.S. will bear watching for the first frost north along and north of I-80, but at this point it doesn’t look to bring a widespread hard freeze to the heart of the Corn Belt. The percentage of corn at maturity and harvested is low but not historically low. In my opinion, the recent excessive rainfall is going to be a bigger concern for maturing crops than the chance of a hard freeze. The good news is that most of next week looks to be sunny and dry, albeit with seasonal temperatures, so that should help things finishing up.