The effect of rainfall amount and timing on annual transpiration in a grazed savanna grassland

The role of precipitation (P) variability on evapotranspiration (ET) and its two components, transpiration (T) and evaporation (E) from savannas, continues to draw significant research interest given its relevance to a number of eco-hydrological applications. Our study reports on six years of measured ET and estimated T and E from a grazed savanna grassland in Welgegund, South Africa. Annual P varied significantly in amount (508 to 672 mm yr−1), with dry years characterized by infrequent early-season rainfall. T was determined using annual water-use efficiency and gross primary production estimates derived from eddy covariance measurements of latent heat flux and net ecosystem CO2 exchange rates. The computed annual T was nearly constant, 331 ± 11 mm yr−1 (T/ET = 0.52), for the four wet years with frequent early wet-season rainfall, whereas annual T was 268 and 175 mm yr−1 during the dry years. Annual T/ET was linearly related to the early wet-season storm frequency. The constancy of annual T during wet years is explained by the moderate water stress of C4 grass and constant annual tree transpiration covering 15 % of the landscape. However, grass transpiration declines during dry spells. Moreover, grasses respond to water availability with a dieback-regrowth pattern, reducing leaf area and transpiration during drought. These changes lead to an anomalous monthly T/ET relation to leaf-area index (LAI). The results highlight the role of the C4 grass layer in the hydrological balance and suggest that the grass response to dry spells and drought is reasonably described by precipitation timing.

Late-seeded cover crops in a semiarid environment: overyielding, dominance and subsequent crop yield

Interest in cover crops is increasing but information is limited on integrating them into crop rotations especially in the relatively short growing season on the northern Great Plains. A 3-yr research project, initiated in 2009 near Mandan, North Dakota, USA, evaluated (1) what impact cover crops may have on subsequent cash crops yields and (2) whether cover crop mixtures are more productive and provide additional benefits compared to cover crop monocultures. The study evaluated 18 different cover crop monocultures and mixtures that were seeded in August following dry pea (Pisum sativum L.). The following year, spring wheat (Triticum aestivum L.), corn (Zea mays L.), soybean (Glycine max L.) and field pea were seeded into the different cover crop treatments and a non-treated control. A lack of timely precipitation in 2009 resulted in a low cover crop yield of 17 g m2 compared to 100 and 77 g m2 in 2008 and 2010, respectively. Subsequent cash crop yield was not affected by late-seeded cover crops. Cool-season cover crop monocultures were more productive than warm-season monocultures and some mixtures in 2008 and 2010. Relative yield total did not differ from one in any cover crop mixture suggesting that overyielding did not occur. Species selection rather than species diversity was the most important contributor to cover crop yield. Cover crops can be grown following short-season cash crops in the northern Great Plains, but precipitation timing and species selection are critical.

Connecting Antarctic Sea Ice and Mid-latitude Precipitation

<div><span>Climatic changes induce many significant changes to long standing weather patterns. These mechanisms interact to drive consequences that may not be immediately obvious. One such connection involves the apparent relationship between polar sea ice extent and mid-latitude precipitation timing and location. This correlation, its mechanisms, and possible influences on weather are decently understood with respect to the Northern Hemisphere. However, the analogous relation for the Southern Hemisphere has been less studied. This provides an opportunity to examine connections between polar conditions and mid-latitude weather.</span></div><div> </div><div><span>We explore the teleconnection between sea ice extent and lower latitude precipitation over the Southern Hemisphere. We investigate this relationship through observations of sea ice coverage using ICESat and ICESat-2 compared with reanalysis data via MERRA-2 in order to understand the variability of sea ice extent and its impact on midlatitude precipitation over the Southern Hemisphere. This study particularly examines the importance of seasonality and regional variations of the relationship.</span></div>

Characterization of snowfall estimated by in situ and ground-based remote-sensing observations at Terra Nova Bay, Victoria Land, Antarctica

Knowledge of the precipitation contribution to the Antarctic surface mass balance is essential for defining the ice-sheet contribution to sea-level rise. Observations of precipitation are sparse over Antarctica, due to harsh environmental conditions. Precipitation during the summer months (November–December–January) on four expeditions, 2015–16, 2016–17, 2017–18 and 2018–19, in the Terra Nova Bay area, were monitored using a vertically pointing radar, disdrometer, snow gauge, radiosounding and an automatic weather station installed at the Italian Mario Zucchelli Station. The relationship between radar reflectivity and precipitation rate at the site can be estimated using these instruments jointly. The error in calculated precipitation is up to 40%, mostly dependent on reflectivity variability and disdrometer inability to define the real particle fall velocity. Mean derived summer precipitation is ~55 mm water equivalent but with a large variability. During collocated measurements in 2018–19, corrected snow gauge amounts agree with those derived from the relationship, within the estimated errors. European Centre for the Medium-Range Weather Forecasts (ECMWF) and the Antarctic Mesoscale Prediction System (AMPS) analysis and operational outputs are able to forecast the precipitation timing but do not adequately reproduce quantities during the most intense events, with overestimation for ECMWF and underestimation for AMPS.

The Diurnal Cycle of Precipitation: A Comparison of State-of-the-Art Observations and Models

<p>Representation of the diurnal cycle is a key trial of the ability of models to capture precipitation timing, duration, and intra-daily variations.  The state-of-the-art model simulations from the Coupled Model Intercomparison Project (CMIP6), which are set to inform the upcoming IPCC sixth assessment report, are yet to be compared to the diurnal cycle of precipitation according to observations.  The recently released version 6 of the Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) product provides over 19 years of global-gridded observations (June 2000 - Present).  Such state-of-the-art observations, with inputs from space-borne dual-frequency radar, microwave radiometers, infrared sensors and ground-based gauges, have never been available at 0.1˚ gridding every half hour over such a long period.  This study aims to compare the amplitude and time of maximum precipitation accumulation between IMERG observations and CMIP6 models over an 8-year period (June 2000 – May 2008).  Preliminary results suggest that the CMIP6 models typically underestimate the amplitude of precipitation accumulation over land compared to observations, though there are overestimates in the Amazon and across central Africa.  Furthermore, the CMIP6 models typically lag behind observations in their time of maximum accumulation over land; observations suggest a late evening to night maximum whilst CMIP6 models show a late morning to early afternoon maximum.  The results will be beneficial to improving modelling of precipitation across the globe.</p>