Understanding and Modeling Tropical Grasslands Using Remotely Sensed Fluorescence and Soil Moisture

Seasonal grasslands account for a large area of Earth's land cover. Annual and seasonal changes in these grasslands have profound impacts on Earth's carbon, energy, and water cycles. In tropical grasslands, growth is commonly water-limited and the landscape oscillates between highly productive and unproductive. As the monsoon begins, soils moisten providing dry grasses the water necessary to photosynthesize. However, along with the rain come clouds that obscure satellite products (MODIS fPAR/LAI) that are commonly used to study productivity in these areas. We used solar induced fluorescence (SIF) products from GOSAT, GOME-2, and OCO-2 along with soil moisture products from SMAP to “see through” the clouds to monitor grassland productivity. The Simple Biosphere Model (SiB) is used to simulate the seasonal cycles of vegetation in tropical grasslands. In conjunction with SiB, the remotely sensed SIF and soil moisture observations were used to paint a clearer picture of seasonal productivity in tropical grasslands. Initial results show that SiB underestimates SIF intensity during the rainy season and overestimates SIF intensity during the dry season. Observed SIF also shows a very rapid green-up following the rain onset and a rapid brown-down as rains subside. SiB, however, shows a more gradual browning and greening process. Future work will integrate remotely sensed SIF with SiB to add to our growing knowledge of carbon, water, and energy cycling in tropical grasslands.