In arid and semi-arid ecosystems, biological resources such as water, soil nutrients, and plant biomass fluctuate between high and low abundance. Short periods of high resource availability are often triggered by rainfall events, which, despite overall scarcity, can temporarily meet the resource needs of some biological processes. This review proposes a hierarchy of soil moisture pulse events, with corresponding ecological responses. Small pulses trigger only minor ecological events, while larger pulses trigger more inclusive and significant ecological responses. This framework is based on the observation that many biological state changes require a minimum pulse size to trigger. Response thresholds are determined by an organism's ability to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses affect surface-dwelling organisms with fast response times, while deeper infiltration is needed for higher plants. The review discusses how precipitation, climate, and site factors translate into varying soil moisture pulses. It then explores the hierarchical response of ecosystem processes and the evolutionary background of response thresholds. Finally, it addresses the implications of global change, asking whether the hierarchical view of precipitation effects in ecosystems provides new perspectives on the future of arid/semi-arid lands. The review emphasizes the concept of "pulse size," a complex composite of environmental and ecological conditions. It also describes soil moisture pulse characteristics, including pulse depth and duration, and their ecological significance. The paper highlights the importance of pulse depth in partitioning precipitation between transpiration and soil evaporation, and the role of infiltration and redistribution in extending pulse depth. Overall, the review provides a framework for understanding the complex interactions between precipitation and ecosystem responses in arid and semi-arid environments.In arid and semi-arid ecosystems, biological resources such as water, soil nutrients, and plant biomass fluctuate between high and low abundance. Short periods of high resource availability are often triggered by rainfall events, which, despite overall scarcity, can temporarily meet the resource needs of some biological processes. This review proposes a hierarchy of soil moisture pulse events, with corresponding ecological responses. Small pulses trigger only minor ecological events, while larger pulses trigger more inclusive and significant ecological responses. This framework is based on the observation that many biological state changes require a minimum pulse size to trigger. Response thresholds are determined by an organism's ability to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses affect surface-dwelling organisms with fast response times, while deeper infiltration is needed for higher plants. The review discusses how precipitation, climate, and site factors translate into varying soil moisture pulses. It then explores the hierarchical response of ecosystem processes and the evolutionary background of response thresholds. Finally, it addresses the implications of global change, asking whether the hierarchical view of precipitation effects in ecosystems provides new perspectives on the future of arid/semi-arid lands. The review emphasizes the concept of "pulse size," a complex composite of environmental and ecological conditions. It also describes soil moisture pulse characteristics, including pulse depth and duration, and their ecological significance. The paper highlights the importance of pulse depth in partitioning precipitation between transpiration and soil evaporation, and the role of infiltration and redistribution in extending pulse depth. Overall, the review provides a framework for understanding the complex interactions between precipitation and ecosystem responses in arid and semi-arid environments.