Exploring SMAP and OCO-2 observations to monitor soil moisture control on photosynthetic activity of global drylands and croplands

Sun-induced chlorophyll fluorescence (SIF) has proven valuable as a proxy of terrestrial ecosystem photosynthesis that can be used to study the impact of environmental limiting factors on terrestrial ecosystem productivity. In global drylands and croplands, soil moisture is often a primary limiting factor to plant growth. Here we use the SIF retrievals from the Orbiting Carbon Observatory-2 (OCO-2) and soil moisture (SM) retrievals from the Soil Moisture Active Passive (SMAP) satellites to study the soil moisture control on photosynthetic activity of global drylands and croplands. We primarily analyze the predawn SM control on daily peak photosynthetic activity using the morning SM observations from SMAP and the noon SIF observations from OCO-2. On global drylands (i.e., dry sub-humid, semi-arid, and arid zones) and majority of croplands, soil water content is typically low, moisture in the topsoil is critical for plant growth, and satellite-based SIF and SM observations are expected to have high retrieval accuracy. The SMAP SM retrievals show positive daily relationships with OCO-2 SIF for drylands and croplands of the tropics and Australia, where SM is expected to limit plant growth and concurrent data records are sufficient to make statistical inferences. Negative relationships between SIF and SM were observed in forested areas of mid-latitude dry subhumid zones with high average annual SM for which SIF showed a positive relationship with air temperature. The relationships between SM and SIF follow the expected spatial patterns, positive and stronger in drier and warmer regions whereas weaker and often negative in colder and wetter regions. We find strong evidence that the OCO-2 SIF is accurately capturing monthly SMAP SM dynamics particularly for regions with distinct seasonality of rainfall such as the Sub-Saharan North Africa, Indian subcontinent, and South African countries. These results indicate new opportunities for monitoring dryland and cropland productivity solely using satellite observed SIF and soil moisture data.