Complexity in water and carbon dioxide fluxes following rain pulses in an African savanna

The idea that many processes in arid and semi-arid ecosystems are dormant until activated by a pulse of rainfall, and then decay from a maximum rate as the soil dries, is widely used as a conceptual and mathematical model, but has rarely been evaluated with data. This paper examines soil water, evapotranspiration (ET), and net ecosystem CO2 exchange measured for 5 years at an eddy covariance tower sited in an Acacia–Combretum savanna near Skukuza in the Kruger National Park, South Africa. The analysis characterizes ecosystem flux responses to discrete rain events and evaluates the skill of increasingly complex “pulse models”. Rainfall pulses exert strong control over ecosystem-scale water and CO2 fluxes at this site, but the simplest pulse models do a poor job of characterizing the dynamics of the response. Successful models need to include the time lag between the wetting event and the process peak, which differ for evaporation, photosynthesis and respiration. Adding further complexity, the time lag depends on the prior duration and degree of water stress. ET response is well characterized by a linear function of potential ET and a logistic function of profile-total soil water content, with remaining seasonal variation correlating with vegetation phenological dynamics (leaf area). A 1- to 3-day lag to maximal ET following wetting is a source of hysteresis in the ET response to soil water. Respiration responds to wetting within days, while photosynthesis takes a week or longer to reach its peak if the rainfall was preceded by a long dry spell. Both processes exhibit nonlinear functional responses that vary seasonally. We conclude that a more mechanistic approach than simple pulse modeling is needed to represent daily ecosystem C processes in semiarid savannas

Efficient Split-Radix and Radix-4 DCT Algorithms and Applications

This project developed efficient, self-recursive, and fast split-radix and radix-4 algorithms for the Discrete Cosine Transforms (DCT) based on different boundary conditions. The project also addresses the self-recursive and stable aspects of split-radix and radix-4 DCT algorithms having simple, sparse, and scaled orthogonal factors. Moreover, the developed split-radix and radix-4 algorithms attain the lowest theoretical multiplication complexity and flop counts for 8-point DCT matrices in the literature. Numerical results are presented for the arithmetic complexity comparison of the proposed algorithms with the known fast and stable DCT algorithms. Software implementations have been written based on the proposed DCT algorithms. These results show that the proposed algorithms have attained low arithmetic complexity. The execution time of the proposed algorithms is presented while verifying the connection to the order of the flop counts. It is shown that the execution time of the proposed split-radix and radix-4 algorithms are more efficient than the existing radix-2 DCT algorithms. Finally, spectral analysis has been conducted based on the proposed DCT algorithms

Exogenous N addition enhances the responses of gross primary productivity to individual precipitation events in a temperate grassland

Predicted future shifts in the magnitude and frequency (larger but fewer) of precipitation events and enhanced nitrogen (N) deposition may interact to affect grassland productivity, but the effects of N enrichment on the productivity response to individual precipitation events remain unclear. In this study, we quantified the effects of N addition on the response patterns of gross primary productivity (GPP) to individual precipitation events of different sizes (P(size)) in a temperate grassland in China. The results showed that N enrichment significantly increased the time-integrated amount of GPP in response to an individual precipitation event (GPP(total)), and the N-induced stimulation of GPP increased with increasing P(size). N enrichment rarely affected the duration of the GPP response, but it significantly stimulated the maximum absolute GPP response. Higher foliar N content might play an important role in the N-induced stimulation of GPP. GPP(total) in both the N-addition and control treatments increased linearly with P(size) with similar P(size) intercepts (approximately 5 mm, indicating a similar lower P(size) threshold to stimulate the GPP response) but had a steeper slope under N addition. Our work indicates that the projected larger precipitation events will stimulate grassland productivity, and this stimulation might be amplified by increasing N deposition